12 files changed, 251 insertions, 78 deletions

diff --git a/src/api/wix/WixToolset.Data/ErrorMessages.cs b/src/api/wix/WixToolset.Data/ErrorMessages.cs
index 889d1762..e149d54a 100644
--- a/src/api/wix/WixToolset.Data/ErrorMessages.cs
+++ b/src/api/wix/WixToolset.Data/ErrorMessages.cs
@@ -323,21 +323,6 @@ namespace WixToolset.Data
             return Message(null, Ids.DuplicateSourcesForOutput, "Multiple source files ({0}) have resulted in the same output file '{1}'. This is likely because the source files only differ in extension or path. Rename the source files to avoid this problem.", sourceList, outputFile);
         }
 
-        public static Message DuplicateSymbol(SourceLineNumber sourceLineNumbers, string symbolName)
-        {
-            return Message(sourceLineNumbers, Ids.DuplicateSymbol, "Duplicate symbol '{0}' found. This typically means that an Id is duplicated. Access modifiers (global, library, file, section) cannot prevent these conflicts. Ensure all your identifiers of a given type (Directory, File, etc.) are unique.", symbolName);
-        }
-
-        public static Message DuplicateSymbol(SourceLineNumber sourceLineNumbers, string symbolName, string referencingSourceLineNumber)
-        {
-            return Message(sourceLineNumbers, Ids.DuplicateSymbol, "Duplicate symbol '{0}' referenced by {1}. This typically means that an Id is duplicated. Ensure all your identifiers of a given type (Directory, File, etc.) are unique or use an access modifier to scope the identfier.", symbolName, referencingSourceLineNumber);
-        }
-
-        public static Message DuplicateSymbol2(SourceLineNumber sourceLineNumbers)
-        {
-            return Message(sourceLineNumbers, Ids.DuplicateSymbol2, "Location of symbol related to previous error.");
-        }
-
         public static Message DuplicateTransform(string transform)
         {
             return Message(null, Ids.DuplicateTransform, "The transform {0} was included twice on the command line. Each transform can be applied to a patch only once.", transform);
@@ -2369,8 +2354,6 @@ namespace WixToolset.Data
             InvalidDateTimeFormat = 88,
             MultipleEntrySections = 89,
             MultipleEntrySections2 = 90,
-            DuplicateSymbol = 91,
-            DuplicateSymbol2 = 92,
             MissingEntrySection = 93,
             UnresolvedReference = 94,
             MultiplePrimaryReferences = 95,
diff --git a/src/wix/WixToolset.Core/Link/FindEntrySectionAndLoadSymbolsCommand.cs b/src/wix/WixToolset.Core/Link/FindEntrySectionAndLoadSymbolsCommand.cs
index e614d4eb..f9d6ab59 100644
--- a/src/wix/WixToolset.Core/Link/FindEntrySectionAndLoadSymbolsCommand.cs
+++ b/src/wix/WixToolset.Core/Link/FindEntrySectionAndLoadSymbolsCommand.cs
@@ -36,7 +36,7 @@ namespace WixToolset.Core.Link
         /// <summary>
         /// Gets the collection of possibly conflicting symbols.
         /// </summary>
-        public IEnumerable<SymbolWithSection> PossibleConflicts { get; private set; }
+        public IReadOnlyCollection<SymbolWithSection> PossibleConflicts { get; private set; }
 
         /// <summary>
         /// Gets the collection of redundant symbols that should not be included
@@ -140,9 +140,7 @@ namespace WixToolset.Core.Link
             {
                 if (symbolWithSection.Overrides is null)
                 {
-                    var fullName = symbolWithSection.GetFullName();
-
-                    this.Messaging.Write(LinkerErrors.VirtualSymbolNotFoundForOverride(symbolWithSection.Symbol.SourceLineNumbers, fullName));
+                    this.Messaging.Write(LinkerErrors.VirtualSymbolNotFoundForOverride(symbolWithSection.Symbol));
                 }
             }
 
diff --git a/src/wix/WixToolset.Core/Link/ReportConflictingSymbolsCommand.cs b/src/wix/WixToolset.Core/Link/ReportConflictingSymbolsCommand.cs
index 3a07d366..2851fa60 100644
--- a/src/wix/WixToolset.Core/Link/ReportConflictingSymbolsCommand.cs
+++ b/src/wix/WixToolset.Core/Link/ReportConflictingSymbolsCommand.cs
@@ -9,7 +9,7 @@ namespace WixToolset.Core.Link
 
     internal class ReportConflictingSymbolsCommand
     {
-        public ReportConflictingSymbolsCommand(IMessaging messaging, IEnumerable<SymbolWithSection> possibleConflicts, IEnumerable<IntermediateSection> resolvedSections)
+        public ReportConflictingSymbolsCommand(IMessaging messaging, IReadOnlyCollection<SymbolWithSection> possibleConflicts, ISet<IntermediateSection> resolvedSections)
         {
             this.Messaging = messaging;
             this.PossibleConflicts = possibleConflicts;
@@ -18,35 +18,62 @@ namespace WixToolset.Core.Link
 
         private IMessaging Messaging { get; }
 
-        private  IEnumerable<SymbolWithSection> PossibleConflicts { get; }
+        private IReadOnlyCollection<SymbolWithSection> PossibleConflicts { get; }
 
-        private IEnumerable<IntermediateSection> ResolvedSections { get; }
+        private ISet<IntermediateSection> ResolvedSections { get; }
 
         public void Execute()
         {
-            // Do a quick check if there are any possibly conflicting symbols that don't come from tables that allow
-            // overriding. Hopefully the symbols with possible conflicts list is usually very short list (empty should
-            // be the most common). If we find any matches, we'll do a more costly check to see if the possible conflicting
+            // Do a quick check if there are any possibly conflicting symbols. Hopefully the symbols with possible conflicts
+            // list is a very short list (empty should be the most common).
+            //
+            // If we have conflicts then we'll do a more costly check to see if the possible conflicting
             // symbols are in sections we actually referenced. From the resulting set, show an error for each duplicate
             // (aka: conflicting) symbol.
-            var illegalDuplicates = this.PossibleConflicts.Where(s => s.Symbol.Definition.Type != SymbolDefinitionType.WixAction && s.Symbol.Definition.Type != SymbolDefinitionType.WixVariable).ToList();
-            if (0 < illegalDuplicates.Count)
+            if (0 < this.PossibleConflicts.Count)
             {
-                var referencedSections = new HashSet<IntermediateSection>(this.ResolvedSections);
-
-                foreach (var referencedDuplicate in illegalDuplicates.Where(s => referencedSections.Contains(s.Section)))
+                foreach (var referencedDuplicate in this.PossibleConflicts.Where(s => this.ResolvedSections.Contains(s.Section)))
                 {
-                    var actuallyReferencedDuplicates = referencedDuplicate.PossiblyConflicts.Where(s => referencedSections.Contains(s.Section)).ToList();
+                    var actuallyReferencedDuplicates = referencedDuplicate.PossiblyConflicts.Where(s => this.ResolvedSections.Contains(s.Section)).ToList();
 
-                    if (actuallyReferencedDuplicates.Any())
+                    if (actuallyReferencedDuplicates.Count > 0)
                     {
-                        var fullName = referencedDuplicate.GetFullName();
+                        var conflicts = actuallyReferencedDuplicates.Append(referencedDuplicate).ToList();
+                        var virtualConflicts = conflicts.Where(s => s.Access == AccessModifier.Virtual).ToList();
+                        var overrideConflicts = conflicts.Where(s => s.Access == AccessModifier.Override).ToList();
+                        var otherConflicts = conflicts.Where(s => s.Access != AccessModifier.Virtual && s.Access != AccessModifier.Override).ToList();
+
+                        IEnumerable<SymbolWithSection> reportDuplicates = actuallyReferencedDuplicates;
+
+                        // If multiple symbols are virtual, use the duplicate virtual symbol message.
+                        if (virtualConflicts.Count > 1)
+                        {
+                            var first = virtualConflicts[0];
+                            var referencingSourceLineNumber = first.DirectReferences.FirstOrDefault()?.SourceLineNumbers;
 
-                        this.Messaging.Write(ErrorMessages.DuplicateSymbol(referencedDuplicate.Symbol.SourceLineNumbers, fullName));
+                            reportDuplicates = virtualConflicts.Skip(1);
+
+                            this.Messaging.Write(LinkerErrors.DuplicateVirtualSymbol(first.Symbol, referencingSourceLineNumber));
+                        }
+                        else if (virtualConflicts.Count == 1 && otherConflicts.Count > 0)
+                        {
+                            var first = otherConflicts[0];
+                            var referencingSourceLineNumber = first.DirectReferences.FirstOrDefault()?.SourceLineNumbers;
+
+                            reportDuplicates = virtualConflicts;
+
+                            this.Messaging.Write(LinkerErrors.VirtualSymbolMustBeOverridden(first.Symbol, referencingSourceLineNumber));
+                        }
+                        else
+                        {
+                            var referencingSourceLineNumber = referencedDuplicate.DirectReferences.FirstOrDefault()?.SourceLineNumbers;
+
+                            this.Messaging.Write(LinkerErrors.DuplicateSymbol(referencedDuplicate.Symbol, referencingSourceLineNumber));
+                        }
 
-                        foreach (var duplicate in actuallyReferencedDuplicates)
+                        foreach (var duplicate in reportDuplicates)
                         {
-                            this.Messaging.Write(ErrorMessages.DuplicateSymbol2(duplicate.Symbol.SourceLineNumbers));
+                            this.Messaging.Write(LinkerErrors.DuplicateSymbol2(duplicate.Symbol));
                         }
                     }
                 }
diff --git a/src/wix/WixToolset.Core/Link/ResolveReferencesCommand.cs b/src/wix/WixToolset.Core/Link/ResolveReferencesCommand.cs
index 0edbf39c..0e7a7a52 100644
--- a/src/wix/WixToolset.Core/Link/ResolveReferencesCommand.cs
+++ b/src/wix/WixToolset.Core/Link/ResolveReferencesCommand.cs
@@ -27,9 +27,9 @@ namespace WixToolset.Core.Link
             this.BuildingMergeModule = (SectionType.Module == entrySection.Type);
         }
 
-        public IEnumerable<SymbolWithSection> ReferencedSymbolWithSections => this.referencedSymbols;
+        public IReadOnlyCollection<SymbolWithSection> ReferencedSymbolWithSections => this.referencedSymbols;
 
-        public IEnumerable<IntermediateSection> ResolvedSections => this.resolvedSections;
+        public ISet<IntermediateSection> ResolvedSections => this.resolvedSections;
 
         private bool BuildingMergeModule { get; }
 
@@ -63,22 +63,25 @@ namespace WixToolset.Core.Link
             // symbols provided.  Then recursively call this method to process the
             // located symbol's section.  All in all this is a very simple depth-first
             // search of the references per-section.
-            foreach (var wixSimpleReferenceRow in section.Symbols.OfType<WixSimpleReferenceSymbol>())
+            foreach (var reference in section.Symbols.OfType<WixSimpleReferenceSymbol>())
             {
                 // If we're building a Merge Module, ignore all references to the Media table
                 // because Merge Modules don't have Media tables.
-                if (this.BuildingMergeModule && wixSimpleReferenceRow.Table == "Media")
+                if (this.BuildingMergeModule && reference.Table == "Media")
                 {
                     continue;
                 }
 
                 // See if the symbol (and any of its duplicates) are appropriately accessible.
-                if (this.symbolsWithSections.TryGetValue(wixSimpleReferenceRow.SymbolicName, out var symbolWithSection))
+                if (this.symbolsWithSections.TryGetValue(reference.SymbolicName, out var symbolWithSection))
                 {
                     var accessible = this.DetermineAccessibleSymbols(section, symbolWithSection);
                     if (accessible.Count == 1)
                     {
                         var accessibleSymbol = accessible[0];
+
+                        accessibleSymbol.AddDirectReference(reference);
+
                         if (this.referencedSymbols.Add(accessibleSymbol) && null != accessibleSymbol.Section)
                         {
                             this.RecursivelyResolveReferences(accessibleSymbol.Section);
@@ -86,32 +89,34 @@ namespace WixToolset.Core.Link
                     }
                     else if (accessible.Count == 0)
                     {
-                        this.Messaging.Write(ErrorMessages.UnresolvedReference(wixSimpleReferenceRow.SourceLineNumbers, wixSimpleReferenceRow.SymbolicName, symbolWithSection.Access));
+                        this.Messaging.Write(ErrorMessages.UnresolvedReference(reference.SourceLineNumbers, reference.SymbolicName, symbolWithSection.Access));
                     }
-                    else // display errors for the duplicate symbols.
+                    else // multiple symbols referenced creates conflicting symbols.
                     {
-                        var accessibleSymbol = accessible[0];
-                        var accessibleFullName = accessibleSymbol.GetFullName();
-                        var referencingSourceLineNumber = wixSimpleReferenceRow.SourceLineNumbers?.ToString();
-
-                        if (String.IsNullOrEmpty(referencingSourceLineNumber))
-                        {
-                            this.Messaging.Write(ErrorMessages.DuplicateSymbol(accessibleSymbol.Symbol.SourceLineNumbers, accessibleFullName));
-                        }
-                        else
+                        // Remember the direct reference to the symbol for the error reporting later,
+                        // but do NOT continue resolving references found in these conflicting symbols.
+                        foreach (var conflict in accessible)
                         {
-                            this.Messaging.Write(ErrorMessages.DuplicateSymbol(accessibleSymbol.Symbol.SourceLineNumbers, accessibleFullName, referencingSourceLineNumber));
-                        }
+                            // This should NEVER happen.
+                            if (!conflict.PossiblyConflicts.Any())
+                            {
+                                throw new InvalidOperationException("If a reference can reference multiple symbols, those symbols MUST have already been recognized as possible conflicts.");
+                            }
 
-                        foreach (var accessibleDuplicate in accessible.Skip(1))
-                        {
-                            this.Messaging.Write(ErrorMessages.DuplicateSymbol2(accessibleDuplicate.Symbol.SourceLineNumbers));
+                            conflict.AddDirectReference(reference);
+
+                            this.referencedSymbols.Add(conflict);
+
+                            if (conflict.Section != null)
+                            {
+                                this.resolvedSections.Add(conflict.Section);
+                            }
                         }
                     }
                 }
                 else
                 {
-                    this.Messaging.Write(ErrorMessages.UnresolvedReference(wixSimpleReferenceRow.SourceLineNumbers, wixSimpleReferenceRow.SymbolicName));
+                    this.Messaging.Write(ErrorMessages.UnresolvedReference(reference.SourceLineNumbers, reference.SymbolicName));
                 }
             }
         }
@@ -133,14 +138,6 @@ namespace WixToolset.Core.Link
 
             foreach (var dupe in symbolWithSection.PossiblyConflicts)
             {
-                //// don't count overridable WixActionSymbols
-                //var symbolAction = symbolWithSection.Symbol as WixActionSymbol;
-                //var dupeAction = dupe.Symbol as WixActionSymbol;
-                //if (symbolAction?.Overridable != dupeAction?.Overridable)
-                //{
-                //    continue;
-                //}
-
                 if (this.AccessibleSymbol(referencingSection, dupe))
                 {
                     accessibleSymbols.Add(dupe);
diff --git a/src/wix/WixToolset.Core/Link/SymbolWithSection.cs b/src/wix/WixToolset.Core/Link/SymbolWithSection.cs
index 979aa44f..5bdf8360 100644
--- a/src/wix/WixToolset.Core/Link/SymbolWithSection.cs
+++ b/src/wix/WixToolset.Core/Link/SymbolWithSection.cs
@@ -6,12 +6,14 @@ namespace WixToolset.Core.Link
     using System.Collections.Generic;
     using System.Linq;
     using WixToolset.Data;
+    using WixToolset.Data.Symbols;
 
     /// <summary>
     /// Symbol with section representing a single unique symbol.
     /// </summary>
     internal class SymbolWithSection
     {
+        private List<WixSimpleReferenceSymbol> directReferences;
         private HashSet<SymbolWithSection> possibleConflicts;
 
         /// <summary>
@@ -46,6 +48,11 @@ namespace WixToolset.Core.Link
         /// <summary>
         /// Gets any duplicates of this symbol with sections that are possible conflicts.
         /// </summary>
+        public IEnumerable<WixSimpleReferenceSymbol> DirectReferences => this.directReferences ?? Enumerable.Empty<WixSimpleReferenceSymbol>();
+
+        /// <summary>
+        /// Gets any duplicates of this symbol with sections that are possible conflicts.
+        /// </summary>
         public IEnumerable<SymbolWithSection> PossiblyConflicts => this.possibleConflicts ?? Enumerable.Empty<SymbolWithSection>();
 
         /// <summary>
@@ -68,6 +75,20 @@ namespace WixToolset.Core.Link
         }
 
         /// <summary>
+        /// Adds a reference that directly points to this symbol.
+        /// </summary>
+        /// <param name="reference">The direct reference.</param>
+        public void AddDirectReference(WixSimpleReferenceSymbol reference)
+        {
+            if (null == this.directReferences)
+            {
+                this.directReferences = new List<WixSimpleReferenceSymbol>();
+            }
+
+            this.directReferences.Add(reference);
+        }
+
+        /// <summary>
         /// Override a virtual symbol.
         /// </summary>
         /// <param name="virtualSymbolWithSection">Virtual symbol with section that is being overridden.</param>
diff --git a/src/wix/WixToolset.Core/LinkerErrors.cs b/src/wix/WixToolset.Core/LinkerErrors.cs
index 78cd76f0..c234087e 100644
--- a/src/wix/WixToolset.Core/LinkerErrors.cs
+++ b/src/wix/WixToolset.Core/LinkerErrors.cs
@@ -11,6 +11,41 @@ namespace WixToolset.Core
             return Message(null, Ids.DuplicateBindPathVariableOnCommandLine, "", argument, bindName, bindValue, collisionValue);
         }
 
+        public static Message DuplicateSymbol(IntermediateSymbol symbol)
+        {
+            return Message(symbol.SourceLineNumbers, Ids.DuplicateSymbol, "Duplicate {0} with identifier '{1}' found. Access modifiers (global, library, file, section) cannot prevent these conflicts. Ensure all your identifiers of a given type (Directory, File, etc.) are unique.", symbol.Definition.Name, symbol.Id.Id);
+        }
+
+        public static Message DuplicateSymbol(IntermediateSymbol symbol, SourceLineNumber referencingSourceLineNumber)
+        {
+            if (referencingSourceLineNumber is null)
+            {
+                return DuplicateSymbol(symbol);
+            }
+
+            return Message(symbol.SourceLineNumbers, Ids.DuplicateSymbol, "Duplicate {0} with identifier '{1}' referenced by {2}. Ensure all your identifiers of a given type (Directory, File, etc.) are unique or use an access modifier to scope the identfier.", symbol.Definition.Name, symbol.Id.Id, referencingSourceLineNumber);
+        }
+
+        public static Message DuplicateVirtualSymbol(IntermediateSymbol symbol)
+        {
+            return Message(symbol.SourceLineNumbers, Ids.DuplicateSymbol, "The virtual {0} with identifier '{1}' is duplicated. Ensure identifiers of a given type (Directory, File, etc.) are unique or did you mean to make one an override for the virtual symbol?", symbol.Definition.Name, symbol.Id.Id);
+        }
+
+        public static Message DuplicateVirtualSymbol(IntermediateSymbol symbol, SourceLineNumber referencingSourceLineNumber)
+        {
+            if (referencingSourceLineNumber is null)
+            {
+                return DuplicateVirtualSymbol(symbol);
+            }
+
+            return Message(symbol.SourceLineNumbers, Ids.DuplicateSymbol, "The virtual {0} with identifier '{1}' is duplicated. Ensure identifiers of a given type (Directory, File, etc.) are unique or did you mean to make one an override for the virtual symbol? Referenced from {2}", symbol.Definition.Name, symbol.Id.Id, referencingSourceLineNumber);
+        }
+
+        public static Message DuplicateSymbol2(IntermediateSymbol symbol)
+        {
+            return Message(symbol.SourceLineNumbers, Ids.DuplicateSymbol2, "Location of symbol related to previous error.");
+        }
+
         public static Message OrphanedPayload(SourceLineNumber sourceLineNumbers, string payloadId)
         {
             return Message(sourceLineNumbers, Ids.OrphanedPayload, "Found orphaned Payload '{0}'. Make sure to reference it from a Package, the BootstrapperApplication, or the Bundle or move it into its own Fragment so it only gets linked in when actually used.", payloadId);
@@ -46,9 +81,34 @@ namespace WixToolset.Core
             return Message(sourceLineNumbers, Ids.UncompressedPayloadInContainer, "The payload '{0}' is uncompressed and cannot be added to container '{1}'. Remove its Compressed attribute and provide a @SourceFile value to allow it to be added to a container.", payloadId, containerId);
         }
 
-        public static Message VirtualSymbolNotFoundForOverride(SourceLineNumber sourceLineNumbers, string id)
+        public static Message VirtualSymbolNotFoundForOverride(IntermediateSymbol symbol)
+        {
+            return Message(symbol.SourceLineNumbers, Ids.VirtualSymbolNotFoundForOverride, "Could not find a virtual symbol to override with the {0} symbol '{1}'. Remove the override access modifier or include the code with the virtual symbol.", symbol.Definition.Name, symbol.Id.Id);
+        }
+
+        public static Message VirtualSymbolNotFoundForOverride(IntermediateSymbol symbol, SourceLineNumber referencingSourceLineNumber)
+        {
+            if (referencingSourceLineNumber is null)
+            {
+                return VirtualSymbolNotFoundForOverride(symbol);
+            }
+
+            return Message(symbol.SourceLineNumbers, Ids.VirtualSymbolNotFoundForOverride, "Could not find a virtual symbol to override with the {0} symbol '{1}'. Remove the override access modifier or include the code with the virtual symbol. Referenced from {2}", symbol.Definition.Name, symbol.Id.Id, referencingSourceLineNumber);
+        }
+
+        public static Message VirtualSymbolMustBeOverridden(IntermediateSymbol symbol)
         {
-            return Message(sourceLineNumbers, Ids.VirtualSymbolNotFoundForOverride, "Did not find virtual symbol for override symbol '{0}'",id);
+            return Message(symbol.SourceLineNumbers, Ids.VirtualSymbolMustBeOverridden, "The {0} symbol '{1}' conflicts with a virtual symbol. Use the 'override' access modifier to override the virtual symbol or use a different Id to avoid the conflict.", symbol.Definition.Name, symbol.Id.Id);
+        }
+
+        public static Message VirtualSymbolMustBeOverridden(IntermediateSymbol symbol, SourceLineNumber referencingSourceLineNumber)
+        {
+            if (referencingSourceLineNumber is null)
+            {
+                return VirtualSymbolMustBeOverridden(symbol);
+            }
+
+            return Message(symbol.SourceLineNumbers, Ids.VirtualSymbolMustBeOverridden, "The {0} symbol '{1}' conflicts with a virtual symbol. Use the 'override' access modifier to override the virtual symbol or use a different Id to avoid the conflict. Referenced from {2}", symbol.Definition.Name, symbol.Id.Id, referencingSourceLineNumber);
         }
 
         private static Message Message(SourceLineNumber sourceLineNumber, Ids id, string format, params object[] args)
@@ -58,6 +118,9 @@ namespace WixToolset.Core
 
         public enum Ids
         {
+            DuplicateSymbol = 91,
+            DuplicateSymbol2 = 92,
+
             OrphanedPayload = 7000,
             PackageInMultipleContainers = 7001,
             PayloadSharedWithBA = 7002,
@@ -67,6 +130,7 @@ namespace WixToolset.Core
             BAContainerCannotContainRemotePayload = 7006,
             DuplicateBindPathVariableOnCommandLine = 7007,
             VirtualSymbolNotFoundForOverride = 7008,
+            VirtualSymbolMustBeOverridden = 7009,
         } // last available is 7099. 7100 is WindowsInstallerBackendWarnings.
     }
 }
diff --git a/src/wix/test/WixToolsetTest.CoreIntegration/AccessModifierFixture.cs b/src/wix/test/WixToolsetTest.CoreIntegration/AccessModifierFixture.cs
index d0e31760..5e40114f 100644
--- a/src/wix/test/WixToolsetTest.CoreIntegration/AccessModifierFixture.cs
+++ b/src/wix/test/WixToolsetTest.CoreIntegration/AccessModifierFixture.cs
@@ -78,8 +78,8 @@ namespace WixToolsetTest.CoreIntegration
             var errors = BuildForFailure("TestData", "AccessModifier", "DuplicateCrossFragmentReference.wxs");
             WixAssert.CompareLineByLine(new[]
             {
-                @"ln 8: Duplicate symbol 'Directory:TestFolder' referenced by <sourceFolder>\DuplicateCrossFragmentReference.wxs(4). This typically means that an Id is duplicated. Ensure all your identifiers of a given type (Directory, File, etc.) are unique or use an access modifier to scope the identfier.",
-                "ln 12: Location of symbol related to previous error."
+                "ln 12: Duplicate Directory with identifier 'TestFolder' referenced by <sourceFolder>\\DuplicateCrossFragmentReference.wxs(4). Ensure all your identifiers of a given type (Directory, File, etc.) are unique or use an access modifier to scope the identfier.",
+                "ln 8: Location of symbol related to previous error."
             }, errors);
         }
 
@@ -89,7 +89,7 @@ namespace WixToolsetTest.CoreIntegration
             var errors = BuildForFailure("TestData", "AccessModifier", "OverrideWithoutVirtualSymbol.wxs");
             WixAssert.CompareLineByLine(new[]
             {
-                "ln 5: Did not find virtual symbol for override symbol 'Directory:TestFolder'",
+                "ln 5: Could not find a virtual symbol to override with the Directory symbol 'TestFolder'. Remove the override access modifier or include the code with the virtual symbol.",
             }, errors);
         }
 
@@ -99,7 +99,40 @@ namespace WixToolsetTest.CoreIntegration
             var errors = BuildForFailure("TestData", "AccessModifier", "DuplicatedOverrideVirtualSymbol.wxs");
             WixAssert.CompareLineByLine(new[]
             {
-                "ln 14: Duplicate symbol 'Directory:TestFolder' found. This typically means that an Id is duplicated. Access modifiers (global, library, file, section) cannot prevent these conflicts. Ensure all your identifiers of a given type (Directory, File, etc.) are unique.",
+                "ln 14: Duplicate Directory with identifier 'TestFolder' found. Access modifiers (global, library, file, section) cannot prevent these conflicts. Ensure all your identifiers of a given type (Directory, File, etc.) are unique.",
+                "ln 6: Location of symbol related to previous error."
+            }, errors);
+        }
+
+        [Fact]
+        public void CannotCompileDuplicatedVirtual()
+        {
+            var errors = BuildForFailure("TestData", "AccessModifier", "DuplicatedVirtualSymbol.wxs");
+            WixAssert.CompareLineByLine(new[]
+            {
+                "ln 6: The virtual Directory with identifier 'TestFolder' is duplicated. Ensure identifiers of a given type (Directory, File, etc.) are unique or did you mean to make one an override for the virtual symbol?",
+                "ln 10: Location of symbol related to previous error."
+            }, errors);
+        }
+
+        [Fact]
+        public void CannotCompilePublicWithVirtualSymbol()
+        {
+            var errors = BuildForFailure("TestData", "AccessModifier", "VirtualSymbolWithoutOverride.wxs");
+            WixAssert.CompareLineByLine(new[]
+            {
+                "ln 9: The Directory symbol 'TestFolder' conflicts with a virtual symbol. Use the 'override' access modifier to override the virtual symbol or use a different Id to avoid the conflict.",
+                "ln 5: Location of symbol related to previous error."
+            }, errors);
+        }
+
+        [Fact]
+        public void CannotCompilePublicAndOverrideWithVirtualSymbol()
+        {
+            var errors = BuildForFailure("TestData", "AccessModifier", "DuplicatePublicOverrideVirtualSymbol.wxs");
+            WixAssert.CompareLineByLine(new[]
+            {
+                "ln 14: Duplicate Directory with identifier 'TestFolder' found. Access modifiers (global, library, file, section) cannot prevent these conflicts. Ensure all your identifiers of a given type (Directory, File, etc.) are unique.",
                 "ln 6: Location of symbol related to previous error."
             }, errors);
         }
diff --git a/src/wix/test/WixToolsetTest.CoreIntegration/RegistryFixture.cs b/src/wix/test/WixToolsetTest.CoreIntegration/RegistryFixture.cs
index 7bf10e3f..9931a45a 100644
--- a/src/wix/test/WixToolsetTest.CoreIntegration/RegistryFixture.cs
+++ b/src/wix/test/WixToolsetTest.CoreIntegration/RegistryFixture.cs
@@ -4,8 +4,8 @@ namespace WixToolsetTest.CoreIntegration
 {
     using System.IO;
     using System.Linq;
-    using WixInternal.TestSupport;
     using WixInternal.Core.TestPackage;
+    using WixInternal.TestSupport;
     using WixToolset.Data;
     using Xunit;
 
@@ -97,8 +97,16 @@ namespace WixToolsetTest.CoreIntegration
                     "-o", msiPath
                 }, out var messages);
 
-                Assert.Equal(2, messages.Where(m => m.Id == (int)ErrorMessages.Ids.DuplicateSymbol).Count());
-                Assert.Equal(2, messages.Where(m => m.Id == (int)ErrorMessages.Ids.DuplicateSymbol2).Count());
+                var errors = messages.Where(m => m.Level == MessageLevel.Error)
+                                     .Select(m => $"ln {m.SourceLineNumbers.LineNumber}: {m}".Replace(baseFolder, "<baseFolder>").Replace(folder, "<sourceFolder>"))
+                                     .ToArray();
+                WixAssert.CompareLineByLine(new[]
+                {
+                    "ln 8: Duplicate Registry with identifier 'regJnkjRU9YGaMJhQOqKmivWKf_VdY' found. Access modifiers (global, library, file, section) cannot prevent these conflicts. Ensure all your identifiers of a given type (Directory, File, etc.) are unique.",
+                    "ln 7: Location of symbol related to previous error.",
+                    "ln 9: Duplicate Registry with identifier 'regJnkjRU9YGaMJhQOqKmivWKf_VdY' found. Access modifiers (global, library, file, section) cannot prevent these conflicts. Ensure all your identifiers of a given type (Directory, File, etc.) are unique.",
+                    "ln 7: Location of symbol related to previous error."
+                }, errors);
             }
         }
 
diff --git a/src/wix/test/WixToolsetTest.CoreIntegration/RollbackBoundaryFixture.cs b/src/wix/test/WixToolsetTest.CoreIntegration/RollbackBoundaryFixture.cs
index 8d8ac801..54375f67 100644
--- a/src/wix/test/WixToolsetTest.CoreIntegration/RollbackBoundaryFixture.cs
+++ b/src/wix/test/WixToolsetTest.CoreIntegration/RollbackBoundaryFixture.cs
@@ -66,7 +66,7 @@ namespace WixToolsetTest.CoreIntegration
 
                 WixAssert.CompareLineByLine(new[]
                 {
-                    "Duplicate symbol 'WixChainItem:collision' found. This typically means that an Id is duplicated. Access modifiers (global, library, file, section) cannot prevent these conflicts. Ensure all your identifiers of a given type (Directory, File, etc.) are unique.",
+                    "Duplicate WixChainItem with identifier 'collision' found. Access modifiers (global, library, file, section) cannot prevent these conflicts. Ensure all your identifiers of a given type (Directory, File, etc.) are unique.",
                     "Location of symbol related to previous error.",
                 }, result.Messages.Select(m => m.ToString()).ToArray());
 
diff --git a/src/wix/test/WixToolsetTest.CoreIntegration/TestData/AccessModifier/DuplicatePublicOverrideVirtualSymbol.wxs b/src/wix/test/WixToolsetTest.CoreIntegration/TestData/AccessModifier/DuplicatePublicOverrideVirtualSymbol.wxs
new file mode 100644
index 00000000..7ecf4445
--- /dev/null
+++ b/src/wix/test/WixToolsetTest.CoreIntegration/TestData/AccessModifier/DuplicatePublicOverrideVirtualSymbol.wxs
@@ -0,0 +1,17 @@
+<?xml version="1.0" encoding="utf-8"?>
+<Wix xmlns="http://wixtoolset.org/schemas/v4/wxs">
+    <Package Name="Duplicated Override Virtual Symbol" Version="1.0.0.0" Manufacturer="Example Corporation" UpgradeCode="047730a5-30fe-4a62-a520-da9381b8226a">
+
+        <StandardDirectory Id="ProgramFilesFolder">
+            <Directory Id="override TestFolder" Name="Override Test Folder" />
+        </StandardDirectory>
+
+        <StandardDirectory Id="ProgramFilesFolder">
+            <Directory Id="virtual TestFolder" Name="Test Folder" />
+        </StandardDirectory>
+
+        <StandardDirectory Id="ProgramFilesFolder">
+            <Directory Id="TestFolder" Name="Collision Test Folder" />
+        </StandardDirectory>
+    </Package>
+</Wix>
diff --git a/src/wix/test/WixToolsetTest.CoreIntegration/TestData/AccessModifier/DuplicatedVirtualSymbol.wxs b/src/wix/test/WixToolsetTest.CoreIntegration/TestData/AccessModifier/DuplicatedVirtualSymbol.wxs
new file mode 100644
index 00000000..e97d0b57
--- /dev/null
+++ b/src/wix/test/WixToolsetTest.CoreIntegration/TestData/AccessModifier/DuplicatedVirtualSymbol.wxs
@@ -0,0 +1,13 @@
+<?xml version="1.0" encoding="utf-8"?>
+<Wix xmlns="http://wixtoolset.org/schemas/v4/wxs">
+    <Package Name="Duplicated Override Virtual Symbol" Version="1.0.0.0" Manufacturer="Example Corporation" UpgradeCode="047730a5-30fe-4a62-a520-da9381b8226a">
+
+        <StandardDirectory Id="ProgramFilesFolder">
+            <Directory Id="virtual TestFolder" Name="First Virtual Test Folder" />
+        </StandardDirectory>
+
+        <StandardDirectory Id="ProgramFilesFolder">
+            <Directory Id="virtual TestFolder" Name="Second Virtual Test Folder" />
+        </StandardDirectory>
+    </Package>
+</Wix>
diff --git a/src/wix/test/WixToolsetTest.CoreIntegration/TestData/AccessModifier/VirtualSymbolWithoutOverride.wxs b/src/wix/test/WixToolsetTest.CoreIntegration/TestData/AccessModifier/VirtualSymbolWithoutOverride.wxs
new file mode 100644
index 00000000..3db854f5
--- /dev/null
+++ b/src/wix/test/WixToolsetTest.CoreIntegration/TestData/AccessModifier/VirtualSymbolWithoutOverride.wxs
@@ -0,0 +1,12 @@
+<?xml version="1.0" encoding="utf-8"?>
+<Wix xmlns="http://wixtoolset.org/schemas/v4/wxs">
+    <Package Name="Override Without Virtual Symbol" Version="1.0.0.0" Manufacturer="Example Corporation" UpgradeCode="047730a5-30fe-4a62-a520-da9381b8226a">
+        <StandardDirectory Id="ProgramFilesFolder">
+            <Directory Id="virtual TestFolder" Name="Test Folder" />
+        </StandardDirectory>
+
+        <StandardDirectory Id="ProgramFilesFolder">
+            <Directory Id="TestFolder" Name="Missing Override Access Modifier" />
+        </StandardDirectory>
+    </Package>
+</Wix>