Have you ever tried one of those daily pill separators? That might help if you are forgetting every now and then.
I don't think it's a big deal to miss it every now and then, but it could add up after some time. Just try to avoid doing it again. As for double dosing, I've done that when I missed a dose, but I split my full dose three times a day so our dosages could be very different. I know that taking all three would be a bad thing to do because it's too much at once. Two of them, however, wasn't a big deal for me, but again, I don't know if we have similar doses. I was told when I was on two that I should split them because you get more of an overall effect that way. In any event, just my own experience, and I am not a doctor, so take everything I say with a huge grain of salt.
What's done is done. I wouldn't worry too much about it, but I would try to avoid it in the future.