Yettwasezmel 3 ɣef 5 n yetran

sɛur Firefox user ad21e5 di Oct. 15, 2019 ·

It's OK and seems to find duplicates accurately but needs a way to set default KEEP / DELETE options. Reversing the defaults by hand is impossible with 100s or 1000s of duplicates.

This review is for a previous version of the add-on (1.5). 

sɣuṛ Dillinger (Aneflay) di Oct. 16, 2019 ·

You can set which duplicates you prefer to delete in the options: first found, unread etc.
You can also reverse the search direction.
Just experiment a bit, there probably is a setting that matches your preferences.