You could try deleting and reinstalling the app, other than that, contact FB to report it.
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This is a known and longstanding problem. If you Google for "Deleted messages keep reappearing on fb messenger" you will get lots of hits, including this from The Verge that dates from 2018 :-
I'm sure FB are fully aware so yet another app update is probably required.
I have no affiliation whatsoever with O2 or any subsidiary companies. Comments posted are entirely of my own opinion. This is not Customer Service so we are unable to help with account specific issues. Please select the post that helped you best and mark as the solution. This helps other members in resolving their issues faster. Thank you.
