or maybe at least the posts wouldn’t be replicated, but there would only be one community, and when you visit it, posts for the community are downloaded from your primary instance, as well as other instances based on some configuration that makes sense.
Since email is the common analogy, I would extend that to say that you could be John.Smith@gmail. You might also have John.Smith@outlook. Someone else has John.Smith@yahoo. If you wanted, you could setup a new account John.smith@protonmail, or start your own server and be [email protected]
Communities are the same way.