>>> Paul Sinha has also made occasional references about having seen a contestant before
There's nothing in the general eligibility requirements prohibiting someone who's been on the show before from returning to it at a later date:
https://itv.etribez.com/a/itv/thechase/welcome
(One can't see the full registration form there without commencing the actual registration process. There are usually all sorts of questions about criminal convictions, etc in such forms. There might well also be a question asking if a candidate has been on the show before but answering 'Yes' wouldn't necessarily rule them out from reappearing on it).