Extra nipples or breast tissue is fairly common (1-6% of women) and is a result of incomplete regression of the mammary ridge (milk line) during the development of the embryo before birth.
Supernumerary nipple - wikidoc
Internet Scientific Publications
Extra (supernumerary or accessory) nipples or breast tissue
I have an extra nipple!
PDF] Bilateral Axillary Accessory Breast Tissue Revealed by Pregnancy.
Nipple accessory - Altmeyers Encyclopedia - Department Dermatology
Supernumerary Nipples & Breasts - Goldberg Plastic Surgery
Third Nipple (Supernumerary Nipple): Causes, Types & Removal
Supernumerary nipple and seminoma: Case report and review of polythelia and genitourinary cancers
If a girl has a third nipple, would it develop into a third breast
25 Breastfeeding Teaching Tools ideas
Third Nipple: Incidence, Types, Causes, and Removal