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I can flag a post as a duplicate while it is already has been closed as a duplicate
I can flag a post as duplicate while it is already has been closed as duplicate.
For example, go to this question, this question has been already closed as duplicate by a moderator but I can still flag it as duplicate which I think this is incorrect, since the question has been already handled.
My proposal is just to hide the duplicate field in case of the question is already closed as duplicate.
If there is a better duplicate or another duplicate which you want to get it linked to the question, so I think you should add this in a custom moderator flag explaining why the other question is better to be the duplicate target.