Getting pregnant in one go is scary enough. Getting knocked up on two separate occasions -- as in, getting pregnant while already pregnant -- is enough to make me never go off birth control.
That very thing happened to Kate Hill, an Australian woman who got pregnant during two rounds of unprotected sex occurring 10 days away. She was receiving a hormonal treatment for polycystic ovary syndrome -- a condition which meant she wasn't ovulating.
Naturally, when her twin girls were born, the assumption was that they were actual twins. However, little Charlotte and Olivia are two totally different sizes and weights. They were born together, but were actually the result of an egg being released after already conceiving one of the daughters. The strange phenomenon is actually called superfoetation.
Naturally, Kate's husband called the twin girls a "hole in one," which is yet another reason I never want to get married or have babies.
Best of luck to the two little girls and their parents!