In this case, dual credit and AP are double coverage – it increases the chance of getting subject credit and advanced placement for the course (if the college accepts one but not the other), but does not give double credit.
But since the AP test is like another final exam for the course, wouldn’t normal preparation for final exams translate to the AP test, so that students should not be doing a lot worse of the AP test than on the course’s final exam? Unless the course’s grading standards are more lax than AP scoring standards…