It's not that they are double dosing, its that the doctor is expecting them to be on this stuff 100% of the time but they only feel the need to take it when they are in the library studying or for certain classes, and there's no sense in taking it on the weekends when they are doing laundry or binge drinking (although snorting adderall while drunk is not that uncommon). So you end the month with extra by the time you can refill.