First, two identical people living in separate simulations are just as much separate people as any other separate people are separate people. It doesn't matter if there exist an identical replica somewhere else, the value of this person in particular doesn't decrease tinyest bit. They're distinct but identical.

The problem with this is that preference is for choosing actions, and actions can't be about specific people only, they are about the whole reality. The question of how much you value a person only makes sense in the context of a specific way of combining valuations of individual people into valuations of reality.