Hard empirical evidence seems a bit skimpy, but, taking things for what they are at the moment:
An extremely rare set of twins with different fathers has been born in Vietnam, according to a scientist in the country.
Prof Le Dinh Luong, president of the Genetic Association of Vietnam, said DNA testing at his Hanoi lab had confirmed the twins had different fathers, the first such case that he knows of in Vietnam. Only seven cases of bi-paternal twins had been reported in the world as of 2011....
Luong said bi-paternal twins could happen if two eggs from the same mother were fertilised by sperm from two different men between one and seven days apart in an ovulation period.