I had a PR for this, but it started to sprawl too much, and became unwieldy.
It's easy to drop duplicate posts from a feed, but the remaining posts need some way to indicate that there's activity in a post that's been hidden.
Likewise, it's easy to dump all the comments from the different posts into one, but then the fact that it's a technical solution to a user problem becomes apparent - the individual rules of each community clearly didn't matter to the person spamming the same across article across multiples of them, but they suddenly become relevant when handling to potential replies.
It felt like I was stuck in a loop - until I knew what to do with posts, I couldn't do anything with comments, and until I knew what to do with comments, I couldn't do anything with posts. It's a problem that needs logically separating out - which could then include new topics, like what the criteria for a cross post should be - but I found it difficult because everything seemed so inter-related.