I love the "Related questions" search after you have typed in a title. This has prevented me from posting a new question many times. While reading a question like http://stackoverflow.com/questions/58640/great-programming-quotes, I got an idea about the same thing for answers. From what I can tell, this question was originally closed due to duplicate answers from people not bothering to read all previous answers.
My suggestion: while you are typing an answer, it will (periodically? and before you can press post?) compare your answer with the other answers on this question and show a list of "Related answers".
I guess this is not applicable everywhere, but I think it could help preventing duplicates on questions with many answers.