But the same communities can exist on multiple sites, and it's confusing to navigate all of that.
I mean, on Reddit the same communities can exist on the same site. I'm a member of r/europe and r/europes, and of r/introvert and r/introverts...
Federation isn't the cause of competing communities. What happens on Reddit is that eventually enough of a mass of people congregate around one sub for a topic, and that becomes the de facto main one. The same thing will happen here.