It used to be regarded as a myth, but today we have cases of super-fecundation in humans.
Superfecundation is the fertilization of two or more ova from the same cycle by sperm from separate acts of s*xual intercourse. Heteropaternal superfecundation occurs when two different males father fraternal twins. The terms are practically equivalent because, though it is speculated that superfecundation by the same father is a common occurrence; it can only be proven to have occurred when there are multiple fathers.
The word which came from the Greeks was probably gotten from cats and dogs, which can bear litters in which each offspring is from a different father. Stray dogs often produce litters in which every puppy has a different father; they look so dramatically different from breed to breed that a discrepancy in paternity was bound to be noticed. It wasn’t however until relatively recently that superfecundation in humans was discovered.
