[Gallagher]

It's when a person has two sets of DNA in their body.

In the womb, two separate eggs have to get released and fertilized in a cycle. Normally, these would make twins, of the not identical sort. But sometimes, for reasons we don't exactly understand, they'll combine to form one perfectly functional person instead.

Sometimes, you can tell. They'll have differences in the color of their skin, or eyes, or hair, or even turn out as a hermaphrodite. But, sometimes you can't tell unless you happen to find out by accident while doing genetic tests. Either way, it's completely natural, and happens in humans and animals... Probably more often than we realize, actually.