Do chemistry majors have a competitive advantage in the medical school admissions process?
According to AAMC statistics for 2010, the acceptance rate among physical sciences majors was 47% (2201 accepted out of 4672 applicants) while the acceptance rate among biological sciences majors was only 43% (9559 accepted out of 22327 applicants). (Exact figures appear to be unavailable but chemistry majors almost certainly comprise the great majority of those categorized by AAMC as "physical science majors".) Physical science majors also did better than Biology majors on the MCAT exam with an average score of 29.8 vs 28.1 for Bio majors.
Conclusion: You might have an almost 10% better chance of getting accepted to medical school as a chemistry major than as a biology major. (Of course, there are other explanations for the above results including the possibility that the physical sciences majors that apply are simply better students.)