When a member is creating new thread, based on data he has entered in title and posts, show him suggestions for already posted threads related to same topic. This will prevent multiple threads on same topic and save everyone's time. I have seen this feature on some forums.
I think this is a lapse from the user creating the thread because if he had first research about the topic or keywords to the post he's intending of before proceeding further, the search function will help to see relevants aspect talked about it, so i wouldn't advise while typing the title or content of the post to be popping up suggestions regarding the same post intended to, member's should understand that we have millions of words that could serve as keyword to each word input but typing a sentence or giving a clue phrase and searching it will help direct the search towards the specific aspect in question than pulling the whole suggestions of words.