https://thenextweb.com/facebook/2018/11/27/facebook-messenger-bug-resurfaces-chats-from-years-ago/

Update (10:08 AM, 27 November 2018): A Facebook spokesperson told TNW it’s fixed the bug:

Earlier today, some people may have experienced Facebook resending older messages. The issue, caused by software updates, has been fully resolved. We’re sorry for any inconvenience.