If you only want to take courses that will help you prepare for medical school (rather than focusing on the Certificate requirements), you do not need to repeat the courses you have already taken in your other undergraduate studies.
However, please be advised that most medical schools have a “shelf life” on courses taken by applicants, and a lot of the time, the expiration period for undergraduate courses is 5-7 years. Therefore, if the medical schools to which you are looking to apply have prerequisite requirements (such as one year of General Chemistry, one year of Physic, etc.) for their applicants, then you will want to have completed these courses within the “shelf life limit” at the time of application. If not, you will probably have to re-take them, if not with UCLA Extension, then another educational institution.
You may also speak with one of our advisors by emailing us: hss@unex.ucla.edu