Usernames are unique only to the instance you registered with. Someone can be CrimsonFlash on a different instance than the one you are on, just like they could be CrimsonFlash@gmail.com or CrimsonFlash@yahoo.com.
There’s no need for you to register at all the instances, just pick one and follow the communities you’d like to.
It helps somewhat to replace “Buffalonian buffalo” with “people”: