This condition is known as an accessory chest.

According to Wikipedia:

Accessory breasts, also known as polymastia, supernumerary breasts, multiple chest syndrome, or mammae erraticae, is the condition of having additional breasts. Extra breasts may appear with or without bosoms or areolae. It is a condition and a form of atavism which is most prevalent 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 breasts, for purely aesthetic reasons.