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That's two seperate issues.

1. the rss feed should include different entries for each version, rss managers generally mark already downloaded entries as downloaded, but you can also mark entries as already downloaded manually so you don't download stuff you already downloaded before.
2. Neither isos nor anime is big on deltas like that so it's generally the entire thing every time. With anime you have the advantage that subs can be seperated so if an episode requires a v2 (a sub with fixes) which some sub groups actually do (or some random does it for them, it depends) they can just throw out a subfile which you just put into your video player or w/e, but it's rare nowadays.

Regardless, RSS feeds are generally not a good tool for deltas anyway because you might just decide to skip a release anyway.