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If there are multiple communities about something you like, why not have that thing you like in them both?
It creates bloat in the feed where your scrolling past the same thing over and over.
More than you normally would anyway
Shouldn't people who are in only one of those communities get to see the thing? Not everyone will be subscribed to them both.
Not if you sort by all. Then you see every community in every instance your instance is federated with.
It can end up in seeing the same post a lot.
Not everyone sorts by all, some people only browse on local, and so on. All things considered, I'd favor having a cool thing in multiple places to get the widest reach possible over considering having to scroll past it again a bad thing.
If you're going to follow and interact with multiple communities, I think using the voting system to encourage good content in each one of them is a cool thing to do (especially in the Fediverse which doesn't do shady algorithm stiff to force unrequested things in front of its users; we users are the only thing keeping the good stuff moving up to the top.)