Pre-existing is insurance-speak for Existing.

My family used to have the word pre-dread, which we knew was redundant, but referred to a decision to worry-a-lot-about-how-awful-it-would-be so that when it happened, if it was only bad, bad would be a pleasant relief.

Mostly applied to family get-togethers after daddy was killed, but it works for most situations...

Actually the more I think of it the less pre- is useful or not redundant, with a verb. Pre-noun, like the pre-dawn hours, is ok. Hmmm. Good point.

I have not yet seen post to mean after.