On Programmers I've been adding a message along these lines to old(ish) questions that have enough answers to be CW if they start getting new answers.
Please read all of the existing answers before posting yours. The question is relatively old and has a reasonable number of answers already. There is a good chance that your answer has already been posted.
I know most people don't read dialogs and just click "OK" to get through them, but if we can stop a few people adding to such questions then it's got to be good right?
The age of the question and the number of existing answers required that trigger this message can be configurable.