The answer to this will depend to some extent upon your record at Duke and your reasons for doing so. If you have chosen to take the bare minimum of science courses required to make application to medical school and then have taken some of those at a less demanding institution, the medical schools might question either your self-confidence, your abilities, or both. However, if you take most of your required courses as a part of a challenging curriculum at Duke, but choose to take a required course at another solid institution for financial, scheduling, etc. reasons, they are not likely to be too concerned. If the course is one that is tested on the MCAT, they may look to your MCAT score to be sure you were well prepared.