I have been thinking about this problem for some time but didn't find good solution.
Since reputation system exists people want to get some reward for their answers. As number of users growth, number of duplicate answers will grow as well.
One of the useful features would be a real-time update on question's answers while you're writing your own answer. It can be implemented in the same way as search, but "search result" should be updated every N seconds/minutes. That way I can clearly see that some one else post similar answer.
Another feature that I find to be useful is merging - as @Unsliced@Unsliced suggested. There must be a way to merge similar answers. However it's not clear who should do it and how to divide reputation points between answering people.