At present if you see a question that may be a duplicate and has an easy answer you can post the answers or post a link to the duplicate question. (Or the few hi-rep users can vote to close it as duplicate, think of normal users here, not the people that read Meta)
It is better for Stack Overflow if a link to the duplicate is posted as a comment, or the user votes to close the question as a duplicate. However, the user gets more rep if he/she posts the easy answer quickly.
I'm thinking of something along the lines of:
First person to post a comment that points to possible duplicate gets some rep if the question is closed as a duplicate and at least 2 of the other closers chose that same dupe question.
Whenever someone votes to close as a duplicate, a “possible duplicate” comment should be posted with a link to the other question, if there is not already a link to it in a comments. (The system now does this)
As Ether said, The removal of reputation earned from answering a question that is later closed as a duplicate would at least remove the negative incentive, but is that enough to correct this behaviour?
Is a “duplicate finder” badge part of the solution?
- A bronze badge for being the first person to vote to close a question as a duplicate with the same “duplicate of” question chosen as most closers choose.
- A silver badge for doing the same 25 times.
Improve tools for closing as duplicate, would also help.