Paul-Dominique... what you're seeing, I think, is a bug that occurs when a user deletes their account.
For some reason, existing posts from that account appear as posted by 'unknown'. More confusingly, replies to those posts appear to be posted as replies to the account of the person that's reading them! So I'm seeing those very same replies to the posts you mention as replies to 'my' posts. And, I assume everyone else is seeing them as replies to 'theirs'.
So no need to worry. You neither have a doppelganger, nor is anyone else seeing replies to posts from you that you didn't make!