If you're retaking because your grades weren't stellar the first time around, it may merit a point of explanation. Especially if you had decent grades, then experienced a sudden drop in academic productivity--these kinds of anomalies usually draw the attention of adcoms.
Of course, you must make sure to explain yourself in such a way that DOES NOT make it sound--for lack of a better description--like a pity-party. Tell them what you've LEARNED from your mistakes. Tell them that you grew as a person and THAT is why you've retaken classes--to PROVE to THEM that you are fully capable of being a successful medical student.
Also, bear in mind that far more complex material, in a much higher volume, is thrown at you in medical school. It's going to be difficult. Because you represent an investment to each medical school you apply to, they want to ensure that the risks taken by accepting each student with a background much like yours is returned favorably in that rigorous environment.
If you were to look at your own history, as an admissions advisor, what explanation would YOU need to have faith in the potential aptitude of that applicant?