From BBC News:
The journal Nature says the twins are identical on their mother’s side, but share only half their genes on their father’s side. They are the result of two sperm cells fertilising a single egg, which then divided to form two embryos – and each sperm contributed genes to each child. Each stage is unlikely, and scientists believe the twins are probably unique.
Normally, twins either develop from the same egg which later splits to form identical twins – who share all their genetic material, or from two separate eggs which are fertilised by two separate sperm. This creates non-identical (fraternal) twins – who share 50% of genetic material. Sometimes, two sperm can fertilise a single egg, but this is only thought to happen in about 1% of human conceptions. Most embryos created this way do not survive.
More here.