fdupes is really nice and fast, but (as far as I remember) it was lacking two features that I needed for my use case, which were 1°/ list duplicate dirs (without listing all of the duplicate sub-contents), and 2°/ being able to identify that all the contents in one dir would be included in another part of the FS (regardless of files/dir structures), which is particularly useful when you have a bigmess/ directory that you progressively sort-out in a clean/ directory. Said differently : fdupes helps to regain space but was not able to help me much to reorganize a messy drive...

This is why I wrote https://github.com/karteum/kindfs (which indexes the fs into an sqlite DB and then enables various ways to process it).