No, the point of federation is that you can interact with other sites/servers from your own since the same protocol is shared. So from some-server.com they visited music@lemmy.world and did their interaction. The same way you can go to the Music community at lemmy.ml ( https://lemmy.world/c/music@lemmy.ml ) without leaving lemmy.world and without having to create a new account. Your comments and interaction will be seen without problems by users of lemmy.ml .

Communities with the same name across different instances are not merged, and the way they are referenced will be different. So, the location of lemmy.world/c/Music can also be referenced as !music@lemmy.world while instead the Music at serverB.whatever be can be referenced as music@serverB.whatever .