Give a specific error when a file type is duplicated in the same file

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -7217,6 +7217,9 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="WRN_DuplicateAnalyzerReference_Title" xml:space="preserve">
     <value>Analyzer reference specified multiple times</value>
   </data>
+  <data name="ERR_FileLocalDuplicateNameInNS" xml:space="preserve">
+    <value>The namespace '{1}' already contains a definition for '{0}' in this file.</value>
+  </data>
   <data name="ERR_UnscopedScoped" xml:space="preserve">
     <value>UnscopedRefAttribute cannot be applied to parameters that have a 'scoped' modifier.</value>
   </data>

src/Compilers/CSharp/Portable/Errors/ErrorCode.cs

@@ -2118,6 +2118,7 @@ internal enum ErrorCode
         WRN_DuplicateAnalyzerReference = 9067,
         ERR_FileTypeNonUniquePath = 9068,
         ERR_FilePathCannotBeConvertedToUtf8 = 9069,
+        ERR_FileLocalDuplicateNameInNS = 9070,
 
         #endregion
 

src/Compilers/CSharp/Portable/Symbols/Source/SourceMemberContainerSymbol.cs

@@ -431,7 +431,10 @@ private DeclarationModifiers MakeAndCheckTypeModifiers(
                         case SymbolKind.Namespace:
                             for (var i = 1; i < partCount; i++)
                             {
-                                diagnostics.Add(ErrorCode.ERR_DuplicateNameInNS, declaration.Declarations[i].NameLocation, this.Name, this.ContainingSymbol);
+                                // note: a declaration with the 'file' modifier will only be grouped with declarations in the same file.
+                                diagnostics.Add((result & DeclarationModifiers.File) != 0
+                                    ? ErrorCode.ERR_FileLocalDuplicateNameInNS
+                                    : ErrorCode.ERR_DuplicateNameInNS, declaration.Declarations[i].NameLocation, this.Name, this.ContainingSymbol);
                                 modifierErrors = true;
                             }
                             break;

src/Compilers/CSharp/Portable/Symbols/Source/SourceNamespaceSymbol.cs

@@ -377,6 +377,9 @@ private static void CheckMembers(NamespaceSymbol @namespace, Dictionary<string,
                             case (SourceNamedTypeSymbol left, SourceNamedTypeSymbol right) when isFileLocalTypeInSeparateFileFrom(left, right) || isFileLocalTypeInSeparateFileFrom(right, left):
                                 // no error
                                 break;
+                            case (SourceNamedTypeSymbol { IsFileLocal: true }, _) or (_, SourceNamedTypeSymbol { IsFileLocal: true }):
+                                diagnostics.Add(ErrorCode.ERR_FileLocalDuplicateNameInNS, symbol.Locations.FirstOrNone(), name, @namespace);
+                                break;
                             case (SourceNamedTypeSymbol { IsPartial: true }, SourceNamedTypeSymbol { IsPartial: true }):
                                 diagnostics.Add(ErrorCode.ERR_PartialTypeKindConflict, symbol.Locations.FirstOrNone(), symbol);
                                 break;