Reward finding duplicate questions - +10, +2, -5
More often than not, even though questions are marked as exact duplicates, they get answered by at least one person. Obviously, we should not prevent people from answering questions marked as a duplicate because there is no way to know for sure (until the community decides for sure) that the question is indeed a duplicate.
Many times, the duplicate questions come in the form of simple questions that have been asked and answered countless times on SO. I believe that some users may even answer these questions knowing that they are dupes for some quick and easy rep.
What if properly marking an answer as a duplicate generated a little rep for the user, assuming that the community agreed the question was a dupe? This would encourage more research on the part of those attempting to answer the question. It may also encourage users in the community to properly flag duplicate question more often (the users the knowingly answer dupe questions don't want to mark them as a dupe because it would encourage others to do so, hence limiting their potential for rep).
Simply put: When flagging an answer as an exact duplicate, the user or users gain rep when there is a consensus.