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For instance, I've voted to close this question as an exact duplicate, but now there are two answers added and one is already accepted. What's the point to close it now?
It the questions are actually duplicates, the answers can be merged into one question. The acceptance on the closed question will be lost, and the OP on the original question can accept one of the merged answers if they wish.
Closing the question means no more answers will be allowed to it. If it is closed as a duplicate, there is also a link where a potential additional answer may be placed.