Steps to reproduce:

1. Used Thunderbird to access a Gmail or Google Workspace account.
2. Noticed that emails in the "All Mail" folder were also appearing in other folders, leading to duplicates in Thunderbird global search results.
3. Identified that this issue arises because Gmail’s "All Mail" folder aggregates emails from all other folders (including Sent, Inbox and Label), causing duplicates in email clients like Thunderbird that treat "All Mail" as a separate folder. The only case where there is no duplicate is when an email has been archived (as it then is not part of any other folder).

Actual results:

• Thunderbird displayed duplicate emails in search results, one from the "All Mail" folder and others from their respective folders.
• This led to clutter and confusion in the user interface, making email management less efficient.

Expected results:

• Thunderbird should ideally recognize and filter out duplicate emails originating from Gmail's "All Mail" folder when returning search results.
• If an email is present in the "All Mail" folder and another folder, only the email from that other folder should show be displayed, regardless of its presence in "All Mail" and other folders.
• The solution should work universally, irrespective of the language settings in Gmail, by using a method to identify the "All Mail" folder through attributes or flags instead of the folder name.
• An optimized algorithm should be employed for checking duplicates to ensure efficient performance without significantly impacting Thunderbird's speed or resource usage.

I have written an experimental add-on for this, but it is really something that'd benefit all Thunderbird users.
https://github.com/mayerwin/thunderbird-hide-all-mail-duplicates
It doesn't support all locales as such an add on cannot easily add a new folder flag to recognize the "All Mail" folder (though this information is exposed by Gmail).

The only reason I have to sync the "All Mail" folder is because "archived" emails are not present in any other folder. Unfortunately, this archival folder is also where non archived emails show up :/.
The most ideal scenario would be for the archival to rely on a truly separate label, or for Thunderbird to skip loading emails in the "All Mail" folder when it has already found them in at least another folder (this way, the "All Mail" folder in Thunderbird will only show truly archived emails). This could be an optional mode that'd improve Gmail's usability in Thunderbird.

Betterbird has also implemented a fix exactly for this, that does support all languages:
https://github.com/Betterbird/thunderbird-patches/blob/main/115/features/28-feature-no-gloda-all-mail-dupes.patch
I have tested it and it works very well.