Have you read about panpsychism? It is somewhat aligned with this idea. Rather than say you were someone else in another life, you could also say they were you in this life. We're all the same consciousness in all of the lives, perhaps all at once as you mentioned. We're the expression of the same fundamental force of consciousness.

Akin to saying we are all drops of water in the waves of the ocean, and at one time we were shaped as another drop of water that you know of. This is irrelevant to say because we are all part of the same body of water, and as such, are the same. We are more like features which emerge from it occasionally in various forms, constantly changing was we splash and ripple, eventually sinking back into the ocean we came from.

This isn't strictly what panpsychism is or what it means; there are plenty of hypotheses around what consciousness being a ubiquitous feature of the universe would really mean, with that being just one interpretation.

I really like to think about it. I'm not a panpsychist, but I'm not opposed to the possibilities either. I am a big fan of the idea that we're all made from the same stuff, with the same core "being", perhaps expressed differently through how our physical bodies develop. I can't be upset with people as much if I imagine their circumstance is largely a product of how their physical development played out, and more or less, they are me. Fundamentally they're exactly what I am, expressed slightly differently by forces they largely have no control over.

It's also neat to consider consciousness as a force of the universe, similar to gravity. Why does gravity exist, and what's it for? Well, maybe nothing. Same with consciousness. It's simply there. It's part of how the universe works.

Maybe we're within the atoms of a larger being, and we're conduits of its own consciousness. Like electricity flowing through a circuit to a co-processor. We're the tiny neural network within it, computing all these simulations to help it understand itself better.

Disclaimer: I don't know anything