Although I'm certainly not a fertility expert, I would imagine that they could. Most other animals can and do occasionally have identicle twins, and it would be the same process with a cat.....one ovum completely splits leaving the exact same genes in two fetuses (I don't think that's a word!).
You did say your cat had 7 kittens - than they would actually be Septuplets. Anyway are you asking because 2 of the kittens look alike? If so, that is quite possible with so many of them!