TL;DR: it's an app called "dupd"
An interesting article from 2015 comparing performance of several utilities to find duplicate files:
http://elipapa.github.io/blog/fastest-way-to-find-dups.html (cached version)
More recent version from 2018:
http://www.virkki.com/jyri/articles/index.php/duplicate-finder-performance-2018-edition/ (cached version)