He said gently bro

It’s kind of the entire point of lemmy, it’s decentralized. Nobody can control other instances. There’s nobody “allowing” it including the lemmy devs

I don’t think what you’re describing is technically feasible. If it worked how you describe, then when someone makes a community that instance would have to somehow go and ask every single other instance in existence - hey do you have this community name already? We wouldn’t be able to like, keep a database of all the existing communities everywhere.

these are different communities with the same post in them

You’re welcome to go to a different instance. That’s the entire point of lemmy. If you don’t like how an instance is ran you can go somewhere else.