see, what happens if you tell them that, they start the exact same thread over here, then another mod moves the original thread over here too, and then another mod sees two identical threads (each of which may have garnered a different set of replies), and merges them, with the result that the final thread has all these nuisance posts and the question gets lost
it would be better simply to move the original thread, or push the little red "report bad post" icon next to the post number, and tell a moderator to move it