I suggest rewarding a silver badge to users with <3k reputation for correctly finding and flagging duplicate questions.
The community would benefit having most part of its users engaged to keep the site clean and organized.
Why only users with <3k rep?
Because they are majority and because usually they do a poorer job finding duplicates than other users (both in amount of duplicates found per user, and closing accuracy). Also, it will be more difficult to game the system because flags need to be reviewed in the queue. Moreover, it will be an incentive for newcomers to pursuit a badge that experienced users won't.
We want low and medium rep users, especially 'new users' to do a great job in the site, when it comes to moderation activities. Therefore, it makes sense having more rewards (likewise privileges) specifically designed to these folks.
Why only duplicates (and not all close reasons)?
We already have rewards for flagging and reviewing, but finding duplicates is usually more difficult because it does not solely depend on reviewing the target post, it requires searching within the site and reading other posts to compare with.
While poor and off-topic content is readily and easy to identify, a duplicate can be well written, but still be a duplicate. It will split the content across the site and possibly will deviate future readers from finding the best answer.
Users with >3k reputation will complain not being eligible for the reward; users can game the system by posting duplicate questions and auto flagging them; the close queue will get even more flooded.
However, there is also the bright side: for people who like to review there will be more material to work; for users trying to game the system there can be some barriers like making an auto flagging not eligible for the badge; and about the queue, duplicate flags demand a higher effort. If users don't do it right they will get flag bans, which is good.
Why a silver badge (not bronze or gold)?
Because finding duplicates is not trivial (bronze), but it is not also so difficult as receiving 100 upvotes for a question or answer, for example.
Also, the threshold for finding duplicates should be neither high nor low, because we want a significant amount of users participating and practicing such positive behavior.
So here is the proposal:
Scout (silver): cast 25 duplicate flags on posts which were later closed as duplicates.
I believe it is easy to implement and manage, besides having potential of good return on investment.