Ask HN: Why do new profs think they can teach the same class more than once?

Your question suggests that older profs know that they can't. More generally your question presupposes falsehoods. New profs think they can because, clearly, they can; it happens every year.

Err, why would they not be able to teach the same class to a new cohort the next time it's taught? Yeah, it's a little repetitive, but how interesting can you make intro-to-$SCIENCE. Profs, usually junior profs, have been doing this at uni for ever.