I'm thinking she's not..

Well, despite having been confirmed bred at least twice, I do not think my kitty is pregnant. She's gone into heat twice since, and since she's supposed to be in her seventh week, I'd think she would, you know, be showing, even if she was only going to have one kitten, but she's not. She's still soooo skinny!

I'll wait another few weeks just to be sure, but I think it's time she went to the vet and got spayed. ^_^

If she has come into heat and isn't getting bigger, she most likely isn't pregnant. I wouldn't wait a few weeks though, I would have her spayed now and feel lucky she didn't get pregnant. I am not sure why you let her breed twice and if you were trying to breed her, but unless she is a registered purebred with breeding rights in a breeding program she should be spayed.
If she is so skinny it would be harmful for her to breed now. I glad she isn't pregnant, please don't wait your luck will run out, so get her done now, and she will be much happier and safer!