Of the vote-to-close reasons, marking as "duplicate" seems unique in that:
- It immediately results in the OP getting his question answered
- It frequently requires knowledge of the subject comparable to what it would take to answer the question directly as well as requiring knowledge of the SO repository or ability/willingness to search
- It involves forgoing reputation which would otherwise likely accrue to the person identifying the duplicate
Similarly, adding an additional vote with the same dup target seems akin to "upvoting" an existing answer.
As such, what would be the effects of incentivizing the first mark-as-duplicate voter with something equivalent to an "accept" or "upvote" rep gain once the question is closed, even if a smaller amount (e.g. 5 points)?