Your counselors are correct. You won't have an advantage in the admissions process if you take AP exams on top of your IB assessments. If anything, it might make you seem a little too focused on standardized test scores. The only reasons you might do this are if you're applying to a college that awards credits for AP scores but not IB scores or if you're particularly interested in an AP subject that's not included in the IB curriculum. Otherwise, it's really not necessary or practical - your energy is better spent on improving your application in other ways.