This kind of stuff is relatively common. More twins are conceived than are born. The unborn ones are "reabsorbed" to provide nutrition for the more viable twin. Sometimes left-over body parts are "born" along with the live baby - medical staff tend to whip those away quickly and not tell the mother. One theory of the origins of left-handedness is that the "right-handed" twin doesn't make it, leaving the left-handed twin to be born.
We more often hear about it in animals - the lamb with six legs, or puppy with two tails, and so on.
Peter the Great's Kunstkammer in St. Petersburg is full of oddities that are quite stomach-turning but compelling. We live in a more sanitised age now where we don't like to be upset by these things.