Actually, this sort of mutation has nothing to do with genetics, as one would expect with marrying close relatives--the genes are absolutely fine. What happens is two individuals are conceived, as twins. but instead of separating completely into two separate developing individuals, the early zygote sticks together, and while one twin develops normally, the second attached twin develops as a "parasite", sharing some of the organs of the main twin, but having some organs of its own. In this case the baby has not been evaluated, but I'd guess they share most of their organs, only having individual faces. It is sort of an extreme version of conjoined twins. In fact, there are cases where a person doesn't even know they have a parasitic twin until they have some belly pain or something, get diagnosed with a tumor, and at surgery they discover the tumor is actually a parasitic twin.
Perhaps the reason we hear more about the Indian or other "third world" cases is that they are brought to wealthy nations for surgery. There are no more incidences of incomplete splitting of twins there than anywhere else. It's just that in India, the population is predominantly Hindu, a polytheistic religion worshipping some gods with multiple limbs etc. So, babies born this way are held in high esteem instead of shunned like they are here.
You won't get that worship and publicity from "paki's", since they are mostly Muslim.
I have several friends from that region of the world in arranged marriages--all their kids have the appropriate number of limbs. Keep up the bigotry and I'll start talking about even more shocking medical anomalies until you stop.
