Scrape the marinade from the duck breasts into the remaining marinade and reserve. Pat the duck breasts dry, season them with salt and pepper and set them in a large skillet, skin side down. Cook over moderately high heat for 1 minute. Reduce the heat to low and cook until the skin is well browned, about 6 minutes. Turn the duck breasts and cook until medium-rare, about 3 minutes longer; transfer to a carving board and let rest for 5 minutes.