Suppose, question B is identified as duplicate of question A. Now user X answers B. It might be voted &/or accepted by OP of question B. If this is done knowing full well that post A has an answer already.
Is it fair practice to be followed by all?
As noted in comments, you are discussing about following post:
does the soul go somewhere when we deep sleep?
As soon as I saw the Qn, I linked with the duplicate. Despite that, the user decided to post an answer after 45 minutes!
This special scenario is already discussed in the main meta:
If another question/answer *perfectly* answers a question (but the question is quite different), is it appropriate to answer with just a link?
In a scenario where the questions aren't the same, but the new (potentially duplicate) question is solidly answered by the original question (and/or its answers), it would be appropriate to close as a duplicate. ...
Ethically if there is a repeat value in an answer, then it should be wiki-fied as soon as it's posted to disallow new reputation.
Since, here the answering user is also a moderator, we can't expect any action. But in case of the normal users, we should flag such posts.
BG 3.21 - Whatever a superior person does, another person does that very thing! Whatever he upholds as authority, an ordinary person follows that.