<p>According to <a href="http://meta.stackexchange.com/questions/156409/review-queue-duplicate-automatic-comments">/review queue - Duplicate automatic comments</a> if the comment already exists it is supposed to automatically upvote the existing one.  (Though as the link above showed, there may be some bugs in that feature.) So I think you should go ahead and recommend deletion and not worry about what other users have thought.</p>