Give an incentive for finding duplicate questions
One of the problems with Stack Overflow is the large number of duplicate questions. Just yesterday, I answered a question that I'm sure has been asked before, but a search didn't find a suitable dupe in a reasonable amount of time. I chose to answer the question instead, and got one measly upvote so far (and, the asker accepted an answer that I don't think solves the asker's actual problem, but maybe that's a different issue).
Some questions get a few votes as a duplicate but not quite enough to close, then the 5 day close vote limit expires which wipes out the existing votes, and all that's left is the automatically added duplicate comment. The chances of such a question actually being closed drop precipitously after the first few hours after asking, and then pretty much to zero after five days.
What if there were some way for users to gain a small amount of reputation for identifying a question as a duplicate? Currently, there is no (reputation-based) incentive to do so, and so it relies on users whose sole motivation is the intangible goal of helping to make the site tidier.
Any mechanism for awarding points to users for identifying duplicates would, of course, need some kind of check to make sure that the user has correctly identified a duplicate. Is the automatically added "possible duplicate of..." comment sufficient, since it is attributed to the closing user? Do two users have to agree on a duplicate, or a majority of the closing votes? Many questions can be marked as duplicates of multiple questions, each providing an excellent answer to part of the question. How much rep would closing users be awarded?