According to this interview with Alex Trebek promoting The Jeopardy! Book: The Answers, the Questions, the Facts, and the Stories of the Greatest Game Show in History:
"We save all unused material, although sometimes an answer that isn't
used one day will turn up later under a different category."
Billy Wisse, the head writer for Jeopardy!, says the same in an interview with The AV Club:
AVC: How many questions get thrown out upon review?
BW: Well, in every category one is going to get thrown out, and then
there’s always an extra there for emergencies or technical problems.
And so two clues out of every seven get thrown out, but they could get
used later if you write that same category again or you really like
that clue and you want to write around it. So a lot of clues get
thrown out, but some of them come back again. But we have thousands
and thousands and thousands of unused clues, and most of them will
never see the light of day.