It's called superfoetation - when someone conceives then conceives again between two weeks and a month later.
It's extremely rare in humans. This is only the sixth time it's happened in 100 years.
Fertility expert Professor Simon Fishel says: "It ought not to happen, but it does."
"The first case was reported in 1865 and there have been odd ones every now and again over the decade."
Most of us assume that once a woman becomes pregnant then that's it, but not according to the man who delivered the first IVF baby in 1978.
