Twin black kittens in a litter of five?

How common is it to see kittens so identical that you truly can't tell them apart? They are nine weeks old, and one was slightly smaller than the other at first, but now they are identical in every way we can see. No markings. Identical eyes. Very large ears and very long legs and tail. Their three siblings are nearly identical gray silky (almost long hair but not quite) with blue-green eyes. I've never seen kittens this gorgeous. Their mother looks very much like the two black kittens: very long slender legs and tail, large ears, but her eyes are more gold than green. The gray kittens are almost impossible to tell apart too. Has anyone heard of this sort of consistency in a mixed breed litter?