This Meta question: Give an incentive for finding duplicate questions deals with various ways of rewarding the finding of duplicates.
This concrete suggestion: Reward finding duplicate questions - +10, +2, -5 was declined as too extreme - from what I gathered, it is out of the question to the SE team to even think about subtracting reputation from "bad" closevoters. (A notion which I personally think may be misguided. But well.)
So, a more modest suggestion.
The first user to come up with a duplicate suggestion is rewarded with 5 reputation points if the question ends up getting closed with their suggestion.
The user gets to keep those reputation points in any case, even if the question is later reopened.
Subsequent users voting for the closing are not rewarded.
To prevent abuse, a duplicate suggestion must be voted on at least 4 times to make the closing final. If there are three different duplicate suggestions, one of them must garner at least four votes. Yes, that would increase the number of closevotes needed in some cases - but you can make the argument it should be that way under the current system, too.
To prevent abuse by people mass suggesting fraudulent duplicates, there would have to be limits - for example, voting rights could be restricted if you have a significant number of duplicate votes that no one agreed with, and a moderator flag raised eventually with the option of suspending users with egregious records.