I think it is pretty unlikely that anyone is going to share souls. There is no way to show that. People used to think that Henry was the reincarnated version of Henry senior, Regina’s father, but the same problem applies here. There is no way of showing that they have the same souls and what would be the point of doing that. Even the idea of a soul seems weird in this world where everyone just goes to another place, the underworld, the good place, seemingly fully intact as themselves.
I think this is simply all magic comes with a price and this was the price of this bit of magic. Lucy’s life for Anastasia.