Siam Twin Baby:
The Process of ovum fission could be overcome before the ovum being fertilized, so that can yield the twin baby. In the case of baby Siam twin can caused by fission process happened imperfect and ovum already fertilized.
This happening also just happen on the story below:
A baby with two faces was born in a northern Indian village, where she is doing well and is being worshipped as the reincarnation of a Hindu goddess, her father said Tuesday.
The baby, Lali, apparently has an extremely rare condition known as craniofacial duplication, where a single head has two faces. Except for her ears, all of Lali's facial features are duplicated -- she has two noses, two pairs of lips and two pairs of eyes.
"My daughter is fine -- like any other child," said Vinod Singh, 23, a poor farm worker