Czkawka, dupeGuru, and VisiPics are my go-to for non-exact photo duplicates. I typically run all three, since they don't all find the same duplicates. VisiPics runs fine under wine.

But Czkawka's real strength is non-exact video duplicates. The only other tool I've found that does that is videoduplicatefinder.

Czkawka has something for finding duplicate music, though I've never used it.