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.