Semi-identical twins -- a boy and girl who are identical on their mother's side but share only 78% of their father's genome -- have been identified in Brisbane, Australia. This is only the second known case, ever. From the BBC:
"The mother's ultrasound at six weeks showed a single placenta and positioning of amniotic sacs that indicated she was expecting identical twins," (said Prof Nicholas Fisk whose team at the Royal Brisbane and Women's Hospita cared for the children when they were born in 2014).
"However, an ultrasound at 14 weeks showed the twins were male and female, which is not possible for identical twins."
If one egg is fertilised by two sperm, it results in three sets of chromosomes, rather than the standard two - one from the mother and two from the father.
And, according to researchers, three sets of chromosomes are "typically incompatible with life and embryos do not usually survive".
The identity of the twins has not been revealed.
A scientific paper about these rare humans was published the New England Journal of Medicine: "Molecular Support for Heterogonesis Resulting in Sesquizygotic Twinning"