I don’t know enough specifically about the technology yet, but maybe it’s something that needs to be handled by either the clients or a third layer that does integration. On usenet, you had essentially the same info going to every server, with some servers disallowing some newsgroups. Many banned binaries, for example, to reduce resource use and avoid piracy.

I could see merging comment chains together if the user is subscribed to each of the services and the article is identical.

I also think that the problem is exacerbated ironically by the lack of posting at the reddit level. Between having too few posts made and too few users to upvote them, and getting the sorting algorithms sorted out, any new posts get pushed to the top.