Heat a large skillet. Using a sharp knife, score the duck breast skin in a cross-hatch pattern. Season the breasts with salt and pepper and add 3 to the skillet, skin side down. Cook over moderate heat until the skin is deep brown and crisp, about 5 minutes; spoon off the fat once or twice. Turn the breasts and cook until browned on the bottom, about 2 minutes. Transfer the breasts to a baking sheet, skin side down, pour off the fat from the skillet and repeat with the remaining breasts. Bake the duck in the oven for about 6 minutes, or until medium rare. Transfer to a carving board, skin side up, cover loosely with foil and let rest for 5 minutes.