The AP English exam (if I remember correctly) has a really clear-cut rubric: part of that rubric, to get above a 4, is that you must cite at least two of the primary sources provided (again, I may be misremembering, it's been four years, and this may actually be the case for AP US History, instead). Which would mean, unless they also cite a different, third source, citing the same source twice cannot earn above a 4. Even if the rest of the paper is excellent.
Still, the fact that the grading scale places an unwarranted amount of emphasis on citation doesn't make it arbitrary or less concrete, so I think my point still stands. Students taking the exam should know that rubric inside and out, so I think it's fair even though the requirement isn't directly related to writing. Arbitrary requirements are part of life. Students also have to wear clothes during the exam even though you don't need clothes to write; I'm sure many great authors have done their best work in a state of dress that would be completely unacceptable in an AP exam.
