I agree with you, but in my experience, if the answer is an answer, it will not be removed, unless it meets some other criteria for removal (not an answer, should be a comment, link-only, etc).
Example: I flagged an answer to this question:
Return FileName Only when using OpenFileDialog
You can see how there is an accepted answer already, but some users start drifting in only recently (years later, mind you) and posting either the exact same answer or a slight variation of the accepted answer.
They're technically not wrong, just (in my opinion) piling on.
I asked the mods if they could lock the question because it looks like it was turning into a honeypot for new users. Flag was rejected because even though they are late, they are answers. As much as I don't like that behavior, I agree with the policy.
So unless an answer is not an answer, simply removing it because it is a late duplicate probably isn't going to happen.
As a suggestion, when there are already answers to a question, there might be some kind of popup or addition to an existing textbox (if there is one, I haven't asked a question in a while) that says something like:
Is there already an answer that answers the question? Please consider upvoting that answer or editing it to add more information instead of posting the same answer.