According to Wikipedia:
Accessory bréasts, also known as polymastia, supernumerary bréasts, multiple chest syndrome, or mammae érraticae, is the condition of having additional bréasts. Extra bréasts may appear with or without bósoms or areolae. It is a condition and a form of atavism which is most prevalént in male humans, and often goes untreated as it is mostly harmless. In recent years, many affected women have had a plastic surgery operation to remove the additional bréasts, for purely aésthetic reasons.