While the language is unfamiliar, I think I am seeing folder names on the right of that image. and they are different, so I do not think Duplicate is quite the correct term.

Matt said

While the language is unfamiliar, I think I am seeing folder names on the right of that image. and they are different, so I do not think Duplicate is quite the correct term.

THUNDERBIRD allows to link one message into different folders. So message is still duplicate. How to fix this? No sense to display one message several times.

It's slavic and in a form of Cyrillic.

If I translate it as Ukrainian (hinted by the "i" character), I get these folder names:

incoming All mail

So, it's a gmail account, and the user is apparently exposing the All Mail folder. So I'd say, of course it is duplicated. You're showing two folders which don't map on to each other by name, but one is a subset of the other in terms of its contents.

I don't know how you get the All Mail folder to be shown in Unified Folders, unless you added it by hand.

I never synchronize or display the All Mail folders in my gmail accounts. What's the point?

In any way this is duplicating. Same mails have to excluded from duplicating.

For example TheBat doesn't have this bug.

This is a "feature" of Gmail under IMAP. I really doubt the bat! or any other mail client will behave in a significantly different manner using the IMAP protocol. My guess it you used the Bat! as a POP client.

There is duplicates - see attached GIF. After I mark one email as read - all duplicates also will mark as read. So Thunderbird display same mail multiple times.

Display one mail multiple times is a bad practice. Thunderbird must have an option to hide duplicate mails.