Does doubling up count me towards four years of math?

If you have fulfilled your HS requirements and are up to AP CALC AB already and AB stats you don't need to continue unless you want to. It depends on what your intended major is where you are applying. If you are going to be a math or physics major and applying to CalTech and MIT, then take AP CALC BC. But also remember that MIT doesn't give you college credit for your STEM APs only your humanities APs. If you get in an enroll you have to take their Core STEM requirements all over again. As a rising Senior, you have limited time, less than 6 months before all your applications are due, and emailed, so just remember that what is most important is that you have all your bases covered on your application. If you have a weakness in your essays, recommendations, ECs, grades or course rigor, it's important to use the 6 months to fill that gap or strengthen that part of your profile. Review all your requirements and be honest about what you think your weaknesses are and work on them. For example if what I've listed is not an issue but you think you are a poor interviewer, work on that, it will be more important that you present the best version of yourself.

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So as far as I know there are looking for 8 semesters of math or if a semester of math equals 5 credits they would look for 40 credits and you would have achieved that this year though going over it never hurts.