>>17470vichan does md5 hashes (through the optional .json API) and lynxchan uses I'm guessing SHA256 for filenames. That said, easily evaded with single pixel changes or by some who overlay a link in the image.
phashes could be effective.
I made a prototype system for collective antispam, never got put into active use but technically it could, with a bit of refactoring and improvement.
https://xj9k.neocities.org/ (code not included, but it's nothing advanced. can share if someone gives a fuck)
Fair warning: there are no images and links are broken on purpose, but most of them are CP link spam.
It worked by downloading the catalogs/overboards of vichan and lynxchan boards, looking for duplicate threads that are on multiple sites, generating a vichan filter to block it, and then uploading it onto that site. Just ran on a shitty pi so it's simple.
Of course there are false positives but it gives an insight into commercial spam and evangelical schizos.
I actually got mail from some kiddo whining about free speech because of this lol