Same structures as ovaries, with slightly different receptors that are formed by and respond to different genetic instructions and hormones. Since in females, switching off of egg development and release from month to month works well, we have two of these in the template. In males, they become testicles and form sperm. Having redundancy in reproductive organs helps ensure your offspring 1. happen, and 2. pass on the genes to keep two of these.

Mammals, eh? At least we didn't keep the more than one pair of nipples.