This has nothing to do with federation, reddit was a single system and had the exact same issue. It's simply a result of having communities that overlap in topics, or articles that are relevant for multiple communities.
The key is that each of these posts has a different discussion due to the different community it is in, isn't that good?
Sort of. Here the problem is doubled; the same post doesn’t just go to different communities on the same server but also the same type of community on different servers.
How is that doubled? It's still just "more than one community exists for a thing." I don't see a difference whether or not those communities are on different servers.
Note if you cross post using the built in cross post feature only one will show up in your feed. In this case there is also the fact that beehaw is defederated from lemmy.world