Every now and again, I find someone has posted an answer to a new question that points to an older question that is an exact duplicate. In other words, the question is a duplicate, but the poster has chosen to point this out with an answer, rather than a comment. There is usually no other content in the answer.
What is the correct way to handle these? Are they cheap attempts to gain rep, which should really be comments? Or are they valid answers in their own right which deserve upvotes?
Most of the time, I see this from users who have less than 3k reputation, who presumably would vote to close if they could.
Links like that really should be provided in comments, or even a CW answer if the user has less than the required reputation to post comments.
A comment on the answer stating the protocol will usually get their attention pretty quickly.
They should be closed (or cast the vote, if you have the rep). If you don't have the reputation, flag them.
When I see users flag them with less than 3,000 reputation:
When I see it from users with greater than 3,000 reputation that couldn't be bothered to vote to close the question?
I see it as being somewhere in between ignorance of the system (posting comments as answers) and blatant rep gaming.
Some of them are probably genuinely trying to be of help. On the other hand, it's cheap rep (or perceived as such) to link to an answer that you already know is correct/helpful on account of upvotes or accepts.
I don't worry too much about the user's rep; the question is going to get closed and buried pretty quickly, and possibly even deleted if it's really an exact duplicate, which means they'll lose any ill-gotten rep on a recalc. And in practice, these answers normally don't pick up many upvotes, because most users, even newer users, recognize this as basically phoning it in.
On the other hand, if the answer is really cheap - as in, nothing more than a link and maybe a "dupe" comment - then I'll probably downvote the answer, not because I'm concerned about rep-gaming, but because it's really not a useful answer. It's no better or worse than an answer linking to a CodeProject page or blog post without any sort of explanation about why it's relevant or how it solves the problem. The idea behind Stack Overflow - at least as I understand it - is for users to add their own content, and posting a bare link doesn't quite meet the bar for originality.
Best not to concern yourself with the answerer's motivation and just vote based on the actual quality of the answer. Then maybe leave a comment to the effect that these types of answers are frowned upon in the community.
I typically down vote them, add a comment saying "This should be a comment" and vote to close the question as an exact duplicate.
I'm presuming (your wording of the question is throwing me a bit) that the answer is linking to an answer on a duplicate of the original question.
For new questions and answers, I think the other answers here cover how to deal with them.
However, you may come across a lot of answers like this for old questions from back in the infancy of SO. This was because the option to leave comments hadn't yet been added to SO, so answers were the only way to point people to duplicates. For these "legacy" answers, I usually just leave them alone, but you could probably leave a comment or flag it for the mods saying it should be made CW or deleted and put as a comment now.
When the essence of my answer is a link to another SO answer, I ususally just make it community wiki. Because such answer really "belong to community"
