Fixed issue with duplicate using directives.

ICSharpCode.NRefactory.CSharp/TypeSystem/ResolvedUsingScope.cs

@@ -97,7 +97,7 @@ namespace ICSharpCode.NRefactory.CSharp.TypeSystem
 					CSharpResolver resolver = new CSharpResolver(parentContext.WithUsingScope(this));
 					foreach (var u in usingScope.Usings) {
 						INamespace ns = u.ResolveNamespace(resolver);
-						if (ns != null)
+						if (ns != null && !result.Contains(ns))
 							result.Add(ns);
 					}
 					return LazyInit.GetOrSet(ref this.usings, new ReadOnlyCollection<INamespace>(result));