It only matters in that a person died. A person with their own subjective experience that they no longer get to experience. It doesn’t matter that in this case I inherited their memories, and it doesn’t matter when it happened other than out of curiosity. I’d mourn them the same.

And as for how I would know… If I’m the clone? Obviously I would never have any way to know, short of someone coming up to me. On the other hand if I were the original, I would “know” because I would be dead. (Or rather, I wouldn’t know anything, because the dead don’t experience or think)

Edit: It matters that I inherited their memories in that it might influence the way I see the world, my identity, and their death, but it wouldn’t change the fact that I mourn them. I am a distinct person from other versions of me, regardless of whether I’m a clone or they’re a clone, and if they die it’s just as much a tragedy as any other human death.