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They are among the rarest types of human beings -- so-called "semi-identical" twins.

Now, doctors in Australia say they've identified the world's second known set of these twins, born from an egg fertilized by two sperm.

Semi-identical twins (sesquizygotic) are classed as a third type of twin, in addition to identical and fraternal twins.

The newly identified set o...

A pair of twin toddlers who were once joined at the head are "thriving" more than a year after surgeons used new techniques to separate them.

"We are so grateful and feel so blessed that we get to be their parents and watch them grow and thrive," said the twins' father, Riley Delaney.

The twins' doctors described what it took to separate the girls, who shared brain tissue an...