Czkawka making my CPU earn its living by finding similar images in a folder with thousands of duplicates. Love this tool, it's a must for everyone in this sub. Thank you, /u/krutkrutrar!
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.