I cannot be too precise when this started happening but at some point in the past two months, I have found that Thunderbird repeatedly notifies me about the same unread emails. I think it's probably since upgrading to 91. I am accessing my inboxes with IMAP and they have multiple subfolders. The subfolders may contain 1 or more unread emails. I do not want or need more than one notification about any given unread email. I am quite sure that Thunderbird 78 did not do this, as it is noticeably irritating. I am experiencing this with Thunderbird on Debian accessing my personal email and Thunderbird on Ubuntu accessing my work email.