Yes and yes.
You can have multiple communities with the same name in different instances, and not necessarily about the same subject.
You can also have not only the same display name as multiple other users, but even the same username, if not in the same instance.
As a funny coincidence, your post showed on my home page 2 posts apart from this one:
https://lemmy.world/post/2021471
(Microsoft forcing migration from the default mail app)