Wash duck thoroughly. Rub inside and outside of duck with salt and Chinese five-spice powder. Rub the inside cavity with minced preserved soya bean, then stuff with galangal slices (leave 3 slices for use outside) and garlic cloves. Seal the opening with a bamboo skewer. Let duck stand for at least 2 hours.
Heat enough oil in a big deep wok. Fry the duck until lightly browned. Remove and drain from oil.
Leave 2 tablespoons oil in the wok; add sugar and galangal slices. Heat until it starts to caramelize. Once it turns golden, turn off the heat and add dark soya sauce. Stir to combine.
Add the duck and turn to coat the entire duck with the sauce. Pour in water and add seasoning. Allow the mixture to come to a gentle boil. (Bathe the duck constantly.)
Braise the duck over low heat until meat is tender and sauce is thick. (Make sure that the duck is turned around occasionally while simmering.)
Cut duck into serving pieces and serve with the gravy.