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.

I’ll be staying no matter if Reddit reverses their decision. The communities I’d like to follow might be smaller here, but I’m sure they will grow.