Forbid to implement same same interface twice

Author: IvanGoncharov

spec/Section 3 -- Type System.md

@@ -562,7 +562,7 @@ of rules must be adhered to by every Object type in a GraphQL schema.
             the interface field argument.
       3. The object field may include additional arguments not defined in the
          interface field, but any additional argument must not be required.
-
+5. An object can not implement particular interface more than once.
 
 ### Interfaces