I think its interesting that multiple instances feel they need their own "music@my-instance" community. One might expect that the instance doesn't really matter, and each instance is just a gateway to the larger federated collection of communities.
But that's not really the case - individual instances do have their own flavor to some extent. Moderation policy can be different, instances can avow some kind of purpose, like hexbear.net for instance is explicitly "leftist". Other servers have different priorities.
In that context, it makes more sense to have duplicate communities - for instance if you post a youtube link on on music@hexbear.net you get an annoying warning about youtube. That fits with their leftist perspective. And its likely you'd get a different response from posting leftist music there than posting it on some right wing instance.