I fixed this problem by doing two things. Firstly I deleted
the summary slide at the end of the quiz. This stopped the error
dialog appearing, but the new slides I was trying to insert were
still overwriting an existing question. I then deleted the
questions that were being overwritten, and all subsequent inserted
questions worked correctly. Seems like something got corrupted in
the question slide.
The quiz result summary page gets regenerated so there's no
problem with deleting it if things get corrupted behind the
scenes.