A question that has been closed has been flagged, surely before the question itself was closed. When I check the flagged posts, I notice the question, and the "close question…."
If I click on that button, I get the following error message:
The question should not appear anymore in the flagged posts, once it is closed.
To notice that the question has been reported as closed, when I viewed the "Flagged Posts" page. As the "[closed]" text is not part of the title (just edit a question that has been closed, and you will notice the "[closed]" part doesn't appear in the title for the question you are editing), I get that the software, when rendering the "Flagged Posts" page was able to detect the question was already closed, but it didn't use that information for avoiding to show the "close question…" button, or for avoiding to show the question in the list of the flagged posts.
Removing the question from the flagged posts is probably excessive, but I don't think I would be able to see a flag where the OP selected the "other" option from the question flag dialog; in this case, I don't think there is anything I can do for those flagged posts. If I would be able to see a closed question flagged from a user that thinks the question should be re-opened, then I could vote for re-opening the question.