A post baccalaureate program generally just includes courses that are prerequisites for medical school. A second bachelor's in chemistry or biology will probably take more time and include some classes that aren't required to gain entrance to medical school.
Of you already have a degree, a second is probably of no more worth than a comprehensive post-bacc program. A second degree in biology would've taken me 2 years, but I was able to finish my DIY post-bacc in 1 year.
