To my annoyance, Hands-Free mode plays sentences marked as “Known”. In my opinion, it should ignore them, at least by default.
To verify the current behavior, you can create a custom collection with just two sentences, and mark one as “known”; you will find that both sentences will be played.
I find that the only way to have Hands-Free mode ignore sentences is to mark them “Ignored”, but I don’t want to do that for sentences that are correct.