Yes, but only at the end of a calling from a female. There can be up
to 3-4 days difference between the first mating & the last one, so
consequently, some the kittens are slightly different in size &
maturity. If they are viable, with hard work, they can do very well-
but there are other times when they simply aren't ready to meet the
world, & sadly we loose some. All kittens can in a litter can be Sired
by different tom cats. The only time that you can be sure of who the
father of the kittens are, is if you contain the female in call & mate
her with a specific male.

In the wild, a female cat can't be sure that she will  meet a suitable
male when she happens to call, so what happens is that the female
doesn't actually decided to mate until the right male is around. She
starts calling to the male, & he will try to stick like glue to her.
Ovulation & the production of a feline egg happens whilst mating, or
during copulation. Because the male has a barbed penis, it scrapes the
side of the womb on withdrawal, which stimulates the females body to
release an egg. Male sperm can last for up to around 48 hours, so if
the female has multiple fathers, you simply can't be sure who sired
the kitten unless by fluke they happen to look like the father in some
way.

The exact answer you are looking for is: Can a female be impregnated
whilst already pregnant.?The direct answer is, "Yes, but only whilst
she is calling for more suitors to the litter she is already
impregnated with, during the same call too".

Sheelagh >"o"<