Should french fries be fried two times

Here is an excellent recipe by the guys over at Cooking Issues (and also a follow up report) with almost everything you'd ever want to know about chips. That first recipe is supposedly good even after they've cooled off:

Our standard fries are good even when cold.

Another option is to cook a lot of chips but stop after their first frying and freeze them; when you want to eat them, simply put the frozen chips into the fryer for the second frying. This also has added benefits:

Freezing acts like partial dehydration. When the frozen fries are finished, they liberate water freely, leading to rapid dehydration and good crust formation with a porous interior. Pre-frozen fries are crunchier than fresh and stay crisper longer after they are fried, but they tend toward hollow fry.

You are right that the moisture in the potatoes are what's causing the oil to 'foam'. If you don't dry your potatoes sufficiently they can quite easily cause the oil to 'boil' explosively out of the frier; this is why you never pour water on a deep-fat pan fire.

There are various ways to dry out your chips/straws, but one of the most effective ways to is to lay them out on a tray or plate and put them, uncovered, in the fridge for a couple of hours, making sure there are no strong odours in there first. Not only will this prevent too much foaming, it will result in really crisp chips!