> Ah, right, thanks. I now dimly recall some old project realizing fs-snapshots using hard links, which one could consider some sort of deduplication as well.

Most modern CoW filesystems also allow you to mark two files as duplicates, but without sharing subsequent mutations between them. Rmlint supports that, too.

Btw, I'm working on adding deduplication to Bcachefs, and because it's extent-based and not blockbased, the logic will look a lot more like rmlint than what you described.