yeah, that's kind of an issue. Many communities exist more than once on multiple instances. Then again, this can also be a benefit. Maybe one of them isn't to your liking -> choose another one. Or one of them goes down -> there's a backup.

They're separate but you can connect to communities across instances. I would recommend posting to !newcommunities@lemmy.world to give it some attention so others are aware it exists.

lemmy handles it like email does. The full community name (like your username) has the domain included.

So you can have showerthoughts@lemmy.world and showerthoughts@lemm.ee, and those are totally separate communities.

The same way Reddit handled communities that had nearly identical or similar topics:

One gets popular, the other doesn't.

I am new to Lemmy, but from what I can tell you can create an arbitrary community, as long as the server allows it. Same name on different instances are treated as totally separate entities. In my opinion, as a new user, I think that is highly non optimal as it creates a fragmented set of users for a given topic. If you go to feedit.de and search for technology you will see a number (seems about 10 or so) different communities with the exact same name. It is up to you to go to each one of them and figure out which one, or ones, you want to follow.