As far as I know this isn’t a feature that exists, but I know the protocol should make it fairly easy.
What I’m thinking is for Lemmy to basically have the option to inherit the comment thread when posting a URL to another fediverse service.
E.g. If crossposting a Lemmy thread, you get a tickbox saying “inherit comment section” or something and it makes the new thread effectively a symlink to the original.
This could also be used to bootstrap other fediverse services like pixelfed and peertube by enabling people to comment directly from their Lemmy instance.
The upside is that I don’t scroll down my feed and see the same post 20 times.
I get what you’re saying but surely a lot of these problems are already present. You can ban a user but they can just go create a new account on a different instance and go back to posting wherever they want.
Personally I’d like to see some kind of tracking thing so Lemmy’s aware of duplicate posts and then the client can be configured how to behave according to user preference.