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It totally does prevent it because every community has a unique name, when you include the instance domain. Which is the whole point. The instance is where the community lives.
Ok, but you can still go ahead and create the same community on every instance so you control all the communities with that name.
No! That’s exactly what you can’t do, because if you tried you’d get banned by the admin! In your scenario, there are no admins to stop such a bad actor. But ultimately admins control what communities are on their instance, so you can’t just hijack all communities like that.