Do six-credit courses count once or twice to your GPA?

yes it does count twice. so if you got a 10 in a six-unit course, you would add 20 to your total number of points (CA).

Yeah, the proper way to calculate your average is to multiply your mark in each course by the number of units the course is worth, then divide by the total number of units. If all your courses are the same number of units it doesn't matter, but otherwise, it does make a difference.

GPA = Σ(Grade*Units)/ΣUnits