submission could automatically give search results on the title and/or url before confirming (like SO).
MetaFilter has been doing this for years too and it works ridiculously well. You have to see a list of potential matches before you can commit your post. So +1 to that.
People won't use it. Decent search has been available via SearchYC for years, and no one seemed to use it to prevent duplicate and/or similar submissions. Further, if you submit via the bookmarklet there is a huge incentive not to search.