Suppose a lab technician takes both cells from a zygote that has just twinned and rejoins them. They then grow as though they had never separated and are implanted to yield one healthy baby.
Since, after twinning, there were two babies (by your standard) and yet the final result was that one baby was born, would you say that the lab technician had killed one? Which one? Suppose that the mitosis that produced the original twins occured with a slight mutation in a 'non-functional' part of one cell's DNA. From my understanding, this mutation would occur in about half of the cells of the resulting baby, but not in the other half. So neither twin could really be said to have "died".