Uterine didelphys

Overview
Uterine didelphys is a rare type of deformity (1 in 1000 women) in the female reproductive organs in which some organs may be either split or duplicated. Typically, some of these "extra" organs are non-functional or semi-functional appendages, although on occasion they will be completely function in all normal respects, and often independently.

Notable cases
A UK woman with two wombs gave birth to triplets in 2006. Hannah Kersey, of Northam in Devon, gave birth to a pair of identical twins from an egg that implanted into one womb and then divided, and to an infant from a single egg that implanted into the other womb. This was the first known birth of its kind.