If an answer is recognized as a duplicate, is there a policy about that?
Given two answers to the same question:
If they are textual copies of each other, then this is a plagiarism offence, and it is best handled by a custom moderator flag explaining the situation. If they are not identical but still very similar, this can still be a plagiarism case, handled similarly, though there is a big gray area on where it stops being plagiarism.
On the other hand, it's perfectly possible for two answers to have essentially the same content without being copies of each other. This is all in the game and both answers are in a fair competition for votes; ideally the clearest one should rise to the top.
If you suspect foul play, flag for moderator attention or make your views known in a comment. If the flag is way off base then the mod that handles it will leave appropriate feedback when declining it. It's not a problem to err on the side of flagging unless it's a repetitive failure on the same issue.