Is it possible to have two cavities in one tooth or is the dentist just trying double-charge for filling?

It is possible to have two cavities in one tooth. Teeth have different surfaces, including the chewing surface (occlusal surface), the sides (proximal surfaces), and the root surface. Cavities can form on any of these surfaces, and it is possible to have a cavity on both the occlusal and proximal surfaces of the same tooth. This can happen due to several factors, such as poor oral hygiene, deep pits or fissures on the tooth surface, or a high sugar diet.

If you are concerned about the accuracy of your dental diagnosis, you can seek a second opinion from another dentist. It is important to remember that dentists are professionals who have undergone extensive training and are committed to providing the best possible care for their patients.