The problem you pose has a more immediately-relevant application: What is a good proportion of resources to devote to non-human life? For instance, when is it better to save a non-human species from extinction, than to turn its habitat into farmland to sustain an equal number of humans? We might agree that one human is worth more than one spotted owl; but not that the ten-billionth human is worth more than the last spotted owl. This is because humans are similar to each other. The identity you invoke is just the most extreme case of similarity.

I've mentioned before on LW that the best moral principle I know, where the goodness of a moral principle is measured as something like

1 / [ bits needed to specify moral principle x Kullback-Leibler divergence(distribution of actions governed by the moral principle || distribution of actions that I endorse)]

is that higher informational complexity is better than lower informational complexity. This nicely deals with this problem, as well as with its generalization to cases where the duplicate copies are not exact duplicates, but merely highly correlated.