If a multi-verse does indeed exist, then there can be a completely different version of "US" in a completely different universe.
The idea is that this multiverse is infinite and produces infinite number of bubble universes. We live in one of these bubbles, a universe, that has certain sets of laws that seems "fine tuned" for life as we know it. When speaking of infinity, any probability of an exact replica or similar universe will be probable even if the probability is small.
So the idea that there could be a different version of us is plausible, and looking at probability, for our atoms to build up again to be us again, in an infinite multiverse, that produces many universes like ours, will be a certainty that just awaiting to happen.
So maybe the religious were not so wacko's after all of describing an infinite life beyond ours. Just not in the same form as they imagined it to be.
Even if the multiverse theory was true, it would in no way strengthen the argument that an afterlife exists. What proof would there be that a person's "soul" or consciousness somehow travels to one of these other universes? It's just a downright nonsensical notion that has no proof even if you make up precedents such as this one.