It's never appropriate to post an answer containing just a link.
Users arrive at SE sites to get answers, not links going to somewhere else that might contain an answer. In addition, links can often go bad (for reasons such as the linked post getting deleted) which leaves the answer as useless.
If you want to answer a question with the content of another answer posted to a non-duplicate question, then I'd suggest posting an answer that contains:
- The relevant parts of the other answer in block quotes
- A link to the original source so it can be correctly attributed to the person who originally wrote the answer
- Additional text of your own explaining how the quoted content applies to the specific question if possible
If you want an example, check out a recent answer of mine about how to post a duplicate answer to a non-duplicate question.
And of course if you just want to be lazy, you can just leave a comment containing the link to the answer instead of posting your own answer :)
In regards to whether you should close non-duplicate questions as duplicates just because they have the same answer, no you should not.
To quote one of the SE Employees when I asked him about the subject:
If there are two ways to ask the same question, we've usually said
it's better to leave it open since people might find it either way. –
I agree with this. We should not be closing non-duplicate questions as duplicate just because the answer addresses both questions.