They always told us only 1 sperm could fertilize 1 egg. Well, not this guy's sperm. Two sperm+1egg=semi-identical twins.

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.

...
These twins, who were conceived normally, only came to the attention of scientists because one was born with sexually ambiguous genitalia.

The child was discovered to be a hermaphrodite, and has both ovarian and testicular tissue, while the other child is anatomically male.

But genetic tests show both are "chimeras", and have some male cells - which have an X and Y chromosome, and female cells - which have two X chromosomes.

The most likely explanation for how they were formed is that two sperm cells - one with an X chromosome and one with a Y chromosome - fused with a single egg.

They're so lucky the egg formed 2 embryos instead of staying a single embryo which would have resulted in a child with extra chromosomes. That's usually disastrous. I suppose if that did happen, it would mostly likely have self-aborted.