Dizygotic twins need not concern us, since they each begin as individual zygotes: one ovum, one sperm. That can occur if more than one ovum is released from the ovaries. Monozygotic twins separate between two and ten days after fertilization. So if the concern is that a newly fertilized ovum might split into monozygotic twins in the next few days, and therefore might not yet be a unitary entity, at least we should be able to agree that once that time frame is passed, a unitary entity exists, whether one or two. A distinct human being.
At every point after conception, of course, the offspring, whether single or twin, is separate and distinct from its mother.
It is noteworthy that even identical twins will not have identical DNA. They will both have identical nuclear DNA, but not identical mitochondrial DNA.
Boethius is a worthy philosopher, but not particularly grounded in human embryology and genetics.