Fixed the output of forward references in CLI headers.

We now use a sorted set to prevent duplicated references and have a stable ordering.

src/Generator/Generators/CLI/CLIHeadersTemplate.cs

@@ -97,7 +97,13 @@ namespace CppSharp.Generators.CLI
             // Create a new tree of namespaces out of the type references found.
             var rootNamespace = new TranslationUnit();
 
-            foreach (var typeRef in typeReferences)
+            var sortedRefs = typeReferences.ToList();
+            sortedRefs.Sort((ref1, ref2) =>
+                string.CompareOrdinal(ref1.FowardReference, ref2.FowardReference));
+
+            var forwardRefs = new SortedSet<string>();
+
+            foreach (var typeRef in sortedRefs)
             {
                 if (string.IsNullOrWhiteSpace(typeRef.FowardReference))
                     continue;
@@ -106,6 +112,12 @@ namespace CppSharp.Generators.CLI
                 if (!(declaration.Namespace is Namespace))
                     continue;
 
+                if (!forwardRefs.Add(typeRef.FowardReference))
+                    continue;
+
+                if (typeRef.Include.InHeader)
+                    continue;
+
                 var @namespace = FindCreateNamespace(rootNamespace, declaration);
                 @namespace.TypeReferences.Add(typeRef);
             }