Scientists have revealed details of the world's only known case of "semi-identical" twins.
Twins normally share either half or all their genes
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 were born in the US, but neither their identity or their exact location is being revealed.
Their case is also reported in the journal Human Genetics.
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 on average 50% of their 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.
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.
The twins are now toddlers, and doctors say they are progressing well.
Another case 'unlikely'
Vivienne Souter, a geneticist at the Banner Good Samaritan Medical Center in Phoenix, Arizona who investigated the case, said: "Their similarity is somewhere between identical and fraternal twins.
"It makes me wonder whether the current classification of twins is an oversimplification."
Charles Boklage, an expert on twinning who works at Eastern Carolina University in Greenville, North Carolina, said: "There's value in understanding that this can happen, but it's extremely unlikely that we'll ever see another case."
And David Bonthron, a geneticist at the University of Leeds, said: "The number of these cases is very small, but before they were reported, most people would have said this could never happen."
He added: "Whether these things are academic curiosities, or whether we've overlooked something significant is hard to say.
"A lot of what we know about fertilisation is deductive, because we can't observe these events in humans."