A first step could be to fully resolve cross-posts, then only display if the original hasn't already been shown in the feed. Then display a dropdown of sorts on the original post to reach hidden crossposts.

A second one could be to hash all the things and hide images/videos/... already viewed.

Both these have fundamental downsides. The comments are often the most interesting thing, and if you plan on interacting with them in any capacity, the specific subreddit is important, so hiding that randomly might be undesirable.

It would be nice if there were a multi- service client like a feed reader that organized by unique post and offered the conversations of each service as attributes to it.

I have written something [once] that de-dupes on just the URLs, you could take this way further with images and content analysis.

This is also manly an app to stop me from scrolling across the same stories on HN and lobste.rs again and again multiple times a day. So it lacks reddit specific features.

[once] https://github.com/batram/once