When female cats come into heat, if they mate with more than one male cat there can indeed be multiple fathers to the litter of kittens.
Un-neutered male cats have barbed penises (much like a fishhook), and upon withdrawal, the female cat will often scream (whether from ecstasy or pain is questionable). It is also believed that the barbed penis stimulates ovulation.
Ovulation will usually occur within 20 to 50 hours after mating, and the eggs are viable (capable of being fertilised) for approximately one day. The eggs are fertilised in the oviduct, and then make their way to the uterus via the uterine horn, implanting in the uterine lining within 10 to 12 days.
On the street, a female cat may mate with two or more tom cats over the length of the oestrus cycle - up to 21 days, with an average of seven days. Although genetics of the female cat may come into play, multi-coloured kitten litters will often vividly demonstrate multiple mating.