Remember that its not 16 hours of duty. Its appropriate rest in the last 24 hours. That makes an absolute max "duty" of 16 hours(due to min rest of 8 hours), but there are cases where that "duty" could be restricted to less than 16 hours (comp rest for instance). Try not to think of it as duty, but as rest in the last 24 hours. The FAA makes no regard to "duty".