This varies by school. Some medical schools take the average score. Some schools look at the most recent score. Others take the highest composite score. Nevertheless, it is important to know that if you have taken multiple exams, the schools you apply to will have access to all those scores. If you are taking the MCAT more than once, try to make sure that there is a large gap between your two scores. In the eyes of the admissions committee, a jump from 505 to 512 is more impressive than a jump from 510 to 512.