Some software like digital image processing tools work with sidecar files which are placed next to the original file. E.g. darktable uses
.xmp as file extension, rawtherapee
When you sort out / delete original files outside of the software, you end up with orphaned sidecar files.
find_orphaned_sidecar_files.py helps in such a situation by finds such duplicates.
$ find_orphaned_sidecar_files.py pics pics/dir1/orphaned1.xmp pics/dir2/orphaned2.xmp