Check out the FAQ for the game RISK on the official Hasbro website. Pretty funny stuff. My favorite:
Can I attack the same territory more than once per turn?
Sure. As long as you share a border you can scare the crap out of your neighbors as many times as you like. In fact you can lull them into a false sense of security by wiping out someone else. Then, just when they have started their recovery efforts you can attack what’s left of their pathetic territory. Again. This is a perfect time to unleash your maniacal laughter.