Closed as duplicate nearly 20+ minutes before an answer was posted.
How'd that happen?
The answer was started before the question was closed you say?... Oh no....
Typically.. if you start an answer and the question gets put on hold as you fill in the answer, as soon as you click "submit" you get a message stating answers can't be submitted. (Has happened to me many times.
Also note that my rep is high enough that if the user posted an answer... and deleted it... (which they commonly do) I would have seen the deleted answer when I closed the question. They could have then gone and edited the deleted answer and undeleted. But I saw no such answer when I closed the question.
I'm baffled as to how that answer got posted.