Hey joejoe22,
Generally speaking, medical schools and dental schools love to see improvement. It tells them that yes, you do mess up in the beginning, but you are a diligent pupil because you made up your grade.
You also have to consider the undergraduate program you are in and their minimum requirements to graduate with a bachelor's degree, as most if not all graduate schools list that as a prerequisite of their program.
So in short, if you feel you can do better the second time around, definately retake the class; otherwise your efforts are best spent focusing on your current coursework before going back and hitting the books for any class you might have done average.
Good luck with your studies and take care!