This is interesting in many areas, including the topics of twins, hermaphrodites, and nuances (mistakes?) of conception. An excerpt from a Nature.com article, published March 26, reads:

Researchers have discovered a pair of twins who are identical through their mother’s side, but share only half their genes on their father’s side.
baby1.jpgThe ‘semi-identical’ twins are the result of two sperm cells fusing with a single egg – a previously unreported way for twins to come about, say the team that made the finding. The twins are chimaeras, meaning that their cells are not genetically uniform. Each sperm has contributed genes to each child….

“Their similarity is somewhere between identical and fraternal twins,” says geneticist Vivienne Souter, of the Banner Good Samaritan Medical Center in Phoenix, Arizona. “It makes me wonder whether the current classification of twins is an oversimplification.”
Such twins are probably very rare…. [A]n egg fertilized by two sperm develops into a viable embryo….
“There’s value in understanding that this can happen, but it’s extremely unlikely that we’ll ever see another case,” says Charles Boklage, an expert on twinning who works at Eastern Carolina University in Greenville, North Carolina.
bab2.jpgSouter and her colleagues investigated the twins’ genetic makeup because one was born with ambiguous genitalia. This twin turned out to be a ‘true hermaphrodite’, with both ovarian and testicular tissue. The other twin is anatomically male.
Genetic tests revealed that each twin contained some ‘female’ cells with two X chromosomes, and some ‘male’ cells with an X and a Y. The proportion of each type varies from tissue to tissue in each twin, the researchers report in Human Genetics1.
The babies, now toddlers, were conceived and born normally, and each twin’s growth and mental abilities appear normal….
“The number of these cases is very small, but before they were reported, most people would have said this could never happen,” says geneticist David Bonthron of the University of Leeds, UK.

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