When a dupe is posted, someone usually posts a comment to that effect within minutes. That comment usually gets at least a couple of upvotes while the question is still on the front page. If it's a common dupe, you might see three or four such comments show up at once, all upvoted.
Even with those comments present, though, some people still insist on posting answers. And, invariably, those answers get a few upvotes. I propose eliminating rep gain from answers which are attached to dupe questions.
Edit 1: To be clear, this isn't really about rep-whoring, or rep at all. As @AnonJr put it in a comment, "not gaining rep is the incentive to not answer a known duplicate, not an end unto itself."
- I suspect that some — not all, but certainly some — people answer dupes just for the easy rep. That's the case described in the answers to this question, but on steroids; if it's better to reward someone who answered an hour or two earlier, it must be far better to reward someone who answered a month or two earlier. The potential for copying is certainly there.
- The answers don't add anything to the general fund of knowledge, especially if they're reworded versions of answers posted to the original, as mentioned earlier.
- The presence of answers makes the dupe question appear to provide useful content, which increases users' reluctance to close/delete.
- Knowing that both asking and answering dupes will be futile will be a deterrent to participating in dupes.
- Lowered participation in dupes will reduce SOFU's overall cruft factor.
- Answerers should be dupe-searching before posting anyways, even if it's only checking the "Related" column; it saves time for everyone, most of all the answerers themselves. This will encourage them to do so without being harsh.
- There's an off chance that there might really be new information there, but in that case the answerer can still post the answer to the original question, hence the "semi-pro" status.
- A well-meaning answerer might try to search for a dupe, not find one even though one exists, write and post a decent answer, and be miffed when his work gets him no rep. (FWIW, my search-fu can be weak sometimes, so I can see myself being this well-meaning answerer, but I believe this side-effect is well worth it if it reduces dupe activity.)
While pondering this problem, I came up with two alternate solutions which I thought were not as good; I include them here for completeness.
- Make answerers reload questions before pressing "Post your answer." — Tricky to enforce.
- Delete all answers when a question is closed as a dupe. — A bit more extreme than my actual proposal, with minimal additional value.
Related but not dupe: Display notification when the question you are answering is voted as dupe
Also edit 1: Dupe in an answer: Give an incentive for finding duplicate questions (thanks @Ether!)