The argument is supposed to be self-evident. The argument is reductionist and itself an affront to human dignity. Cloning is unnatural. Nature creates clones all the time, as identical twins. Twins ordinarily think of themselves as individuals, not as carbon copies of someone else.
Identical twins differ in many respects.
An embryo may divide at any time up to about 14 days after conception. Even conjoined twins are different from each other. One was a depressed alcoholic, the other a cheerful teetotaler. They married separate wives, alternated living in separate houses, and fathered 21 children. Abigail and Brittany Hensel share a common body below the neck, but feel sleepy or hungry at different times and may get different grades in school.