As I've said elsewhere, I mostly think that any notion of preserving personal identity (from moment to moment, let alone after my heart stops beating) depends on a willingness to acknowledge some threshold of similarity and/or continuity such that I'm willing to consider anything above that threshold to share that identity.

Defining that threshold such that entities in other universes share my identity means there's lots of me out there, sure. And defining that threshold such that entities elsewhere on Earth right now share my identity means there's lots of me right here.

It's not clear to me why any of that matters.