Flush `Module.ExternalClassTemplateSpecializations` to avoid emitting the same specialization in multiple files

src/Generator/Generators/CSharp/CSharpSources.cs

@@ -149,6 +149,9 @@ namespace CppSharp.Generators.CSharp
                     UnindentAndWriteCloseBrace();
                 PopBlock(NewLineKind.BeforeNextBlock);
             }
+
+            if (Options.GenerationOutputMode == GenerationOutputMode.FilePerUnit)
+                Module.ExternalClassTemplateSpecializations.Clear();
         }
 
         public virtual void GenerateUsings()