VERY IMPORTANT CLARIFICATION: The inbox new message double count only appears on the Thunderbird icon on my taskbar tray. That is, the taskbar icon says new 4 messages when only 2 appear in the inbox. The actual inbox message count in the Thunderbird window is OK Thus, this may be a taskbar icon problem.

Even stranger! The taskbar icon is double counting only one inbox!

Let me explain. My Thunderbird serves two separate email addresses with their own separate inboxes. If inbox A receives two new messages and inbox B receives one new message, the Thunderbird taskbar tray icon will show (red circle number) 5 new messages. In contrast, if inbox A receives one new message and inbox B receives two, the icon will say 4 new messages. A single new message in A is shown on the icon as two; a single new message in B is shown as one. Clearly, the icon new message counter is double counting only inbox A. Very weird! This selective inbox A double counting happens constantly regardless of the combination and number of new messages for each inbox. This happens in real time as new messages come in. To repeat above, every other aspect of Thunderbird email works perfectly fine and the counts of all folders is correct within the Thunderbird window.

My Thunderbird is kept up-to-date and this double counting has been going on for at least 3 months.

In the case of Gmail every new mail appears in the inbox and the all mail folder. That is how they choose to implement IMAP. That is two.