tl;dr: I fully agree with you that there's room for improvement here, but I can't for the life of me decide on a solution I actually like lmao.
Yeah, it's definitely going to be common while the masses settle on which community/instance they want to emphasize.
The question for now is, how do you think this should actually be handled? What counts as identical links? There's several factors that can change, and that makes handling the problem much more complicated than it initially seems.
In your example, it's 1) the same user posting 2) the same link with 3) the same title to 4) different communities. That does seem like basic reposting, and it would initially seem like we should just combine them and have it say "[USER], 10 hours ago posted to [Community1], [C2], [C3], etc"
But each of those communities and each of those instances may abide by different rules. It would definitely be a disservice to each community to pile all the discussion threads into one communal comment section.
So.... I guess if it's 4) a different community, then it's okay to have 'duplicate' threads? Uh oh.
I have a feeling that same argument would apply to veryyy similar threads with 1) different users, 2) different links to the same content, or 3) different titles. So what the heck, how do we improve this situation?? What are we even asking for??