cancelling twice is an ever harder decision than cancelling once... if you feel like you didn't do your best, and that will lead to a score that you will have a hard time living with, i would cancel even if it is your second cancellation.
after all, you do have october ahead of you, without jeopardizing the application process for 2006/2007. if you can live with a score lower than your 'minimum', don't cancel, otherwise, i would.
for me, it was important that my score reflect my best ability, and NOT me screwing up the variables i could control with practice, like keeping calm, preparing well, managing time well, staying focused. i wanted to take the best test i could take, and then let my score be whatever it is. (which leaves me with no excuses once i get the score!) i took dec 04, ran out of time, lost focus, didn't have my head in the game, so i cancelled the score.
i practiced regularly between feb and june, and i retook it yesterday. even if i get the exact same score as i would have in dec (and we shall never know), i feel better about my effort this time, and i know i gave it my best shot. i trained the way i tested, so i tested the way i trained and it made a big difference.
you only get one real shot at this, the question i would ask myself in your shoes is, do i want to settle for less than the best i can do?