As gonnif said, retaking gen chem 1 is an absolute requirement here. Here's the long and the short of it:
1) You must get an A on any retake
- The thought process here is that you've already seen the material once, so you've as good as seen the answer key. If you do that and don't get an A, adcoms will think, "What's the problem here that this guy couldn't even manage an A the second time through?"
2) You should always retake prereqs in which you get lower than a C
- Many med schools require that you get a C or better in any prereq for it to count as "completed" in their book. While some schools may feel differently, it's in your best interest to make yourself eligible to apply to as many schools as possible. That means not disqualifying yourself through failure to complete prereqs to their satisfaction. C- is not a C. And during the retake, refer to point 1 above.
3) Never retake a prereq in which you earned a C or better
- Your AMCAS GPA is calculated using all of your letter grades -- including all the times you took a given course. If you passed a prereq with a C on the first attempt and retake it, both grades will be counted, even if that's not how your university calculates GPA. Remember point 1 above: the expectation is that you'll get an A the second time through, and failure to do so could actually burn you. Better to take a different course instead -- the grade you get will have the same effect on your GPA, but there won't be the absolute expectation of getting an A.
4) Never retake a non-prereq course regardless of your grade
- Again, point 1 above. Reduce your risk. Don't take something if even an A- could get you burned.