Deleted messages reappear in mail
My mail has recently started to bring back messages I've deleted multiple times, in more than one mailbox. I read, answer, delete if I don't want to keep the message, and a few minutes later there it is again. Most reappear as unread; some reappear as read. How can I stop my mailbox from being clogged up this way? I've tried deleting them in my online Gmail account and erasing deleted messages (but they don't turn up in the trash, either.).
iMac 27″, macOS 12.4