Roughly one in every 10,000 chickens is a gynandromorph or dual-sexed chicken. Gynandromorphism has been reported in insects, birds and crustaceans, but not in other species. Although uncommon, in chickens isn’t extremely rare.
Source: Backyard Poultry, March 2019. Link. The current prevailing theory is that bilateral gynandromorphism starts at the very beginning of cell development through polyspermy, when two separate sperms fertilize a single ovum.