Re: Double Grading!
I haven't seen a double grading at my dojo, and I'm guessing we probably won't ever have one, but I have no idea if that's typical. We start at 5th rather than 6th, and it's rare for a beginner to test as early as three months, even with pretty intensive training (I can only remember one), and of course the time between gradings gets longer as you go along.
Another big factor is that we're a fairly small dojo, so it's not uncommon to train for quite some time after you've met the requirements, just because there's no one else who's ready to test. And then every time Sensei starts thinking about testing people, there's always the problem of finding a date that works, which tends to push things off further. So, even within one dojo, I'd say there's a lot of variability. We've had people (well, at least one person) who tested for fifth after one month, we've had many more who trained for over a year before the tested.