It should be possible to change the closing reason or at least remove it.
- Could remove the close vote when the user edits the post
- Could change the closing reason if need be
But the most useful thing is that you could change the question that you marked as duplicate. If I vote and say this question is a duplicate but I misread or the post was edited or I want to change it for whatever reason, I can't. Also, it will automatically put a comment of me saying that this question looks like a duplicate of
x other post so I should be able to change the post it refers too.