I think it's not appropriate that someone just copies someone else's answer. Or even post an answer to a question that is essentially the same as others.
Imagine someone asking: How to do X? And the answers: A1: You do Y to do X. (January 2015, 0 upvotes) A2: You have to do Y to do X. (March 2016, 15 upvotes)
This is just unfair.
This happens in this question: "Error Applying Security - Access is denied" while changing file permissions on windows server 2008
See Cheng Bo's older answer compared to the upvoted Jevunal's answer. Users should not just paste duplicate answers.
My question: what to do in the case? Should I flag? Should moderators delete duplicate answer?