A few weeks ago David B posted The Olsen Twins: MZ or DZ. I have seen several press reports that assert they are "fraternal," and yet they seem to look strikingly alike aside from the fact that one is skeletal. Perhaps they could be the hypothesized polar body twins, that is, a polar body becomes an identical egg, and they are fertilized by two different sperm from the father. In other words, they are "half-identical," and so would resemble each other more than typical fraternal twins.

But there are many possibilities here. Human testicle sizes indicate some level of "sperm competition" and female infidelity, and there are cases of superfecundation, when a woman has more than one sexual partner and different eggs are fertilized by sperm from different fathers.

Theoretically one could imagine a combination of polar body twins conceived via superfecundation. This could result in siblings who are genetically as related as conventional fraternal twins (assuming the fathers are unrelated). One case of superfecundation involved a black woman who had a white and black lover and gave birth to black and mixed-race twins, so the permutations are pretty strange....