Rated 3 out of 5 stars

le Firefox user ad21e5 ar DFómh. 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.

Léirmheas ar leagan níos sine den bhreiseán seo (1.5). 

le Dillinger (Forbróir) ar DFómh. 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.