Rated 5 out of 5 stars

This add-on works great with Thunderbird 78.9.0 on a Mac running Catalina.

Like others, I got zero results at first. Fixes:
- Restart Thunderbird
- Tools > Add-on Options > Remove Duplicate Messages and change filter settings.
Under "General Settings" I left all defaults on. Everything checked. Default action "Move to Trash"
Under "Message Comparison" I kept *everything* checked except Size (important!) and Folder and set "Time Comparison Resolution" to Seconds.

Found 7K duplicates. Created a duplicates local folder and dumped them there. Yay!

This review is for a previous version of the add-on (0.5.1b2). 

Hello Patrick, or whoever else is reading this review. Please open an issue on the repository's GitHub page about this, and perhaps we can get to the bottom of why this restart is necessary.