[CodeActions] Don't return a convert to extension method call code action if the call is already using extension method syntax.

ICSharpCode.NRefactory.CSharp/Refactoring/CodeActions/StaticMethodInvocationToExtensionMethodInvocationAction.cs

@@ -52,7 +52,7 @@ namespace ICSharpCode.NRefactory.CSharp.Refactoring
 			if (invocationRR == null)
 				yield break;
 			var method = invocationRR.Member as IMethod;
-			if (method == null || !method.IsExtensionMethod)
+			if (method == null || !method.IsExtensionMethod || invocationRR.IsExtensionMethodInvocation)
 				yield break;
 			yield return new CodeAction(context.TranslateString("Convert to extension method call"), script => {
 				var newArgumentList = invocation.Arguments.Skip(1).Select(arg => arg.Clone()).ToList();

ICSharpCode.NRefactory.Tests/CSharp/CodeActions/StaticMethodInvocationToExtensionMethodInvocationTests.cs

@@ -136,6 +136,25 @@ class C
 }");
 		}
 
+		[Test]
+		public void IgnoresIfAlreadyExtensionMethodCallSyntax()
+		{
+			TestWrongContext<StaticMethodInvocationToExtensionMethodInvocationAction>(@"
+class A { }
+static class B
+{
+	public static void Ext (this A a, int i);
+}
+class C
+{
+	void F()
+	{
+		A a = new A();
+		a.$Ext (1);
+	}
+}");
+		}
+
 		[Test]
 		public void IgnoresPropertyInvocation()
 		{