This is what I think should happen too - having multiple of the "same community" is a feature, but most of the time, I would suspect most people don't want to create another and would rather join the existing one. If they still want to create another, they'll obviously be able to just hit "No, create community" or whatever (for example, politics@sweden and politics@netherlands would understandably be different despite the same name).

Lemmy has no aggressive internet crawling built in. I'm not opposed to someone integrating a global community searcher within lemmy, that uses some external service, as long as its failure safe and can use a regular search when that service goes down.

Perhaps some kind of "sub-federation" feature allowing communities to automatically share eachother's posts or smthn could be a viable solution?

you wouldn't want to force names to be unique, but offering it as a suggestion, the same way similar posts pop up below when you are creating a post with a similar title/URL, that could be useful!