Give an incentive for finding duplicate questions
When there are exact duplicate questions, you'll sometimes see a lot of people answering it instead of linking to the duplicate and voting to close. Because gaining some rep is better than gaining no rep.
To encourage the behavior of not answering duplicate questions, and instead linking to them, I think it would be a good idea to have a badge that is given out if you are the first to close a closed answer X times. Where X is for example 100 or some other number.
I think encouraging that behavior would help the site a lot.