Sometimes dental schools won't accept AP credit in lieu of taking an actual college class. Therefore, it is perfectly acceptable to actually take a course for which you have already received AP credit. At some schools, you only have your first semester to request that unwanted AP and/or transfer credits get removed (because some institutions charge higher tuition after you reach a certain number of credits and they don't want you to try to lower your tuition when you reach that point). And yes, there are cases where courses cannot be repeated for additional credits, and taking a course for which you already received AP credit counts as repeating a course. In such cases, you will receive a grade, but the institution will figure out a way to ensure that you are not receiving the credit twice.