Suppose that you are captured by aliens who can travel to other universes. Furthermore, they tell you they are going to drop you into a different universe but, and here’s the catch, it is identical to this one. And by identical, I mean in every way, including its history. When you arrive, you find yourself in exactly the same time and place as when the aliens abducted you. There are your socks lying on the floor where you left them. All your books are there, earmarked and underlined with notes, just as if you were the one that marked them. There is your spouse/significant other welcoming you home—kids, neighbors, world, everything is exactly the same. Your photo albums are there with all your pictures of your life just as you lived it. In fact, the aliens tell you that this world is identical to yours in every way, except there is a trout in northern Canada that is 0.5 cm smaller in this place than the same fish in your old universe. That is the only difference, including the neural state of every living being in this universe. Your spouse loves you here and has had the identical history with you that your spouse had back home.
But you know this is not really your spouse. These are not really your parents. These are not your kids. Those are not your socks. This is not your universe (which is still out there somewhere). Yet, all your relationships are in place, just as you left them.
Does it matter? Would you mourn the loss of your other life?