A simple tool to remove duplicate emails from folders. It uses the inbuilt headerMessageId that is generated by every email when created and confirms that any duplicate email has the EXACT same size. This tool does NOT do any nesting or deduplication across folders and will ONLY remove duplicates in EACH selected folder.
Run the "Check for duplicates" command to see what would happen. If you're happy with the results, then run "Delete duplicates" to actually remove duplicates in your folder(s).
You can access the commands via selecting folder(s) and right clicking for the context menu:
Go to the Thunderbird addon store and install: https://addons.thunderbird.net/en-US/thunderbird/addon/duplication-remover/
This addon was shamelessly derived from the MailExtensions repo: https://github.com/thunderbird/webext-examples/tree/master/manifest_v3/menu.