1. Identical twins from one conception are not a single person.
2. An entity with chimerism (two conceptions) is one person, not two people simultaneously.
3. If I make an SCNT clone [1] of someone, that's another person, not property or a mobile body part.
4. They aren't freaking out about zillions of regular miscarriages, because they don't actually believe those are people-deaths.
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[0] They say life, but it really helps to nail them down to a much more specific definition ASAP, because they often to retreat into fallacies of equivocation. HeLa cells are not a person.
[1] https://en.wikipedia.org/wiki/Somatic_cell_nuclear_transfer