A look at third (sometimes called supernumerary) nipples, a relatively common condition where an individual has an extra nipple. Included here are details about causes, diagnosis, and treatment.
Supernumerary Nipple
Some animals have deactivated genes which code for extra appendages. Do humans have any genetic code which is 'switched off' and would lead to different morphology? - Quora
Supernumerary Nipple: Causes, Diagnosis, and Images — DermNet
Accessory Nipple Removal: Cost, Reviews, Safety
Some animals have deactivated genes which code for extra appendages. Do humans have any genetic code which is 'switched off' and would lead to different morphology? - Quora
Supernumerary nipple - wikidoc
Supernumerary nipple dermoscopy
Third Nipple (Supernumerary Nipple): Causes, Types Removal, 47% OFF
Supernumerary Nipple: Causes, Diagnosis, and Treatment