Five years ago, scientists took a cell from a sheep, implanted it into an empty egg and coaxed it to divide as a normal embryo. A perfect genetic copy, Dolly, was the result. But there is some evidence that Dolly aged prematurely. Other clones, from mice to cows, have also proved abnormal.
Twins and even triplets have been around for millenniums, but engineering multiple copies of an adult or a lost child is just too creepy.