Sorry to double reply, but it looks like there's a time limit on editing a post.
The fork of kiwidude's excellent repo for finding duplicates that I made a while back has been updated to (hopefully) work on newer versions of Calibre, and also includes the massive improvements kiwidude made to their code in the meantime.
https://github.com/komali2/calibre-find-duplicates
Remember that this is only possible because of all of their hard work, credit goes to them!