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We always come across questions that are duplicate. If a question already has comment that it is possible duplicate. Then, I always check if that is the correct duplicate and if yes, I flag it as duplicate. But I also see people posting same answers as earlier.
Those answers get upvotes and reputation, even though same answers are already posted in the other questions.
I suggest that after the question is marked duplicate (e.g. after second or third vote while question is still open) answers posted on that question will give less reputation to their author when voted up, e.g. 5 points instead of 10.