Fix serialization source generator error when multiple partials of th…

…e same definition are user-defined (space-wizards#5160)

Robust.Serialization.Generator/Generator.cs

@@ -1,6 +1,4 @@
-using System.Collections.Generic;
-using System.Linq;
-using System.Text;
+using System.Text;
 using Microsoft.CodeAnalysis;
 using Microsoft.CodeAnalysis.CSharp;
 using Microsoft.CodeAnalysis.CSharp.Syntax;
@@ -36,6 +34,7 @@ public void Initialize(IncrementalGeneratorInitializationContext initContext)
                 var (compilation, declarations) = source;
                 var builder = new StringBuilder();
                 var containingTypes = new Stack<INamedTypeSymbol>();
+                var declarationsGenerated = new HashSet<string>();
 
                 foreach (var declaration in declarations)
                 {
@@ -44,6 +43,14 @@ public void Initialize(IncrementalGeneratorInitializationContext initContext)
 
                     var type = compilation.GetSemanticModel(declaration.SyntaxTree).GetDeclaredSymbol(declaration)!;
 
+                    var symbolName = type
+                        .ToDisplayString()
+                        .Replace('<', '{')
+                        .Replace('>', '}');
+
+                    if (!declarationsGenerated.Add(symbolName))
+                        continue;
+
                     var nonPartial = !IsPartial(declaration);
 
                     var namespaceString = type.ContainingNamespace.IsGlobalNamespace
@@ -107,11 +114,6 @@ public void Initialize(IncrementalGeneratorInitializationContext initContext)
 {{containingTypesEnd}}
 """);
 
-                    var symbolName = type
-                        .ToDisplayString()
-                        .Replace('<', '{')
-                        .Replace('>', '}');
-
                     var sourceText = CSharpSyntaxTree
                         .ParseText(builder.ToString())
                         .GetRoot()