Yes, you should. That is not the appropriate action for suggesting a possible duplicate. Until it actually gets closed as a duplicate by 5 people, it is only a possible duplicate and the question is still perfectly valid. The only appropriate action is to vote-to-close and list that as the question believed to be a duplicate, which auto-posts the comment. If they don't have that privilege yet, that's what flagging or commenting is for.
Not to mention, once it does get closed (if it does), Community is just going to add another box on top of that with the properly formatted, actual duplicate box.