Ethics and the generous ontology

According to a view attractive to both metaphysicians and ethicists, every period in a person's life is the life of a being just like that person except that it exists only during that period. These "subpeople" appear to have moral status, and their interests seem to clash with ours: though it may be in some person's interests to sacrifice for tomorrow, it is not in the interests of a subperson coinciding with him only today, who will never benefit from it. Or perhaps there is no clash, and a subperson's interests derive from those of the person it coincides with. But this makes it likely that our own interests derive from those of other beings coinciding with us.