What is a third nipple?

My boyfriend has three nipples. Is he a mutant? - BBC Science Focus Magazine

Third nipple? (pic)

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Third Nipple: Types, Causes, and Removal

Third (supernumerary) nipple: Types, causes, and removal

Third Nipple: Types, Causes, Removal, and More

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Third Nipple (Supernumerary Nipple): Causes, Types Removal, 47% OFF