CloneSpy fucking rules thank you to @tools for the tip, I’ve only scanned 18% of my big drive I use for dumping files and have already cleared up 21GB, fuck yeah
This is very useful, and much faster than many other methods by way of using checksums as part of their comparison!
I'd like to add on another suggestion, since it's not something CloneSpy can do:
If you're needing to try and sort/reorganise/discard duplicate images, even if they're not identical files (non-motion porn collections, unsorted photos, etc), VisiPics is a good tool for the job, as it will gather and display your image duplicates within tolerances you can set, and has many of the same choice options as CloneSpy for filtering selected images. Other suggestions for similar (perhaps newer/better) tools welcome!
I just wish there was something I could use to do the same for gifs or webms/other video formats! If anyone knows a tool or method that would allow batch discovery and selection of duplicate gifs/videos, please let me know!