Utilize the static virtual methods defining in TestDuplication example

Author: BartoszKlonowski

docs/csharp/whats-new/tutorials/snippets/staticinterfaces/Program.cs

@@ -9,6 +9,12 @@
     Console.WriteLine(str++);
 // </TestRepeat>
 
+// <TestDuplication>
+var num = new Duplicate(2);
+num++;
+Console.WriteLine(num.ToString());
+// </TestDuplication>
+
 // <TestAddition>
 var pt = new Point<int>(3, 4);