Thaw duck in refrigerator until partially thawed. The skin should be soft, but the insides should still be frozen. Take duck out of wrapping, and stab the duck 15 to 20 times. Make sure the stab holes go past the fat and into the meat. This is very important for the marinating process.
In a large mixing bowl, mix 1 cup sherry, and 3 tablespoons olive oil. Place duck in the mixing bowl. Spread the sherry and olive oil all over the duck. Cover the mixing bowl, and refrigerate until duck is completely thawed (works best over night).
Preheat oven to 375 degrees F (190 degrees C). Remove duck from marinade, and remove the neck bone and the giblets from inside the duck. Spread some of the marinades inside of the duck. Rub the outside of the duck with 3 tablespoons olive oil and oregano, rosemary, and basil. Pour remaining 1 cup sherry into bottom of roasting pan, and place duck in pan.
Roast for about 30 minutes per 1/2 pound (cooking times may vary.) Baste every 30 to 45 minutes.