If there's another answer added
- much later than an already existing one, and it's
- absolutely the same as another answer, content-wise
then there's rarely need to keep redundant information around.
I wouldn't call out users for posting duplicate answers – it's probably just because they haven't read the other answers or feel the need to add their two cents, but essentially there's:
- No value gained from them
- Less signal, more noise – the opposite of what we want
- Potential reputation gain from something that has already been said, which I find unfair towards users who actually invested time in the original answers
By all means, if a new answer is added and there is nothing in it that hasn't already been said, then I'd flag it for removal. I'd add a custom text like mentioning the other answer as well:
Exactly the same has already been answered by @user
However — and here's where you should be careful — if the answer does contain more information or goes beyond what others have stated, then do not flag it. We encourage new answers at all times, even if a question already has an accepted answer. You can always improve on what is there. You can always phrase something differently, make it easier to understand, or approach a problem from another perspective.
Generally, if answers on some topics seem similar, that's somewhat to be expected. The only case in which I'd flag duplicate answers for removal are those questions that can simply be answered by a single line of code or sequence of commands (which happen to be the same), a folder path, a registry entry, et cetera.
If the question involves argumentation and its answers give more context, then just let those answers stay. Maybe inform the user who answered that something similar has already been posted by pointing them to the other answer(s).
In case of doubt, let it be.
Another edge case is users posting the same answers to multiple questions, thus "duplicates". I know this isn't specifically what you talked about, but this is not the kind of behavior we encourage and I figured I might as well mention that.
These kinds of duplicate answers are a good hint that the questions are actual duplicates and need to be closed. Try to identify which question is the most "canonical" one, and bring them to moderator attention, to Meta Super User or post them in the Vote to Close chat room.