Preheat the oven to gas 6, 200C, fan 180°C Score a crisscross pattern into the duck fat using a sharp knife. Rub the soy sauce into the flesh side of the duck and season.
Heat a heavy frying pan and place the duck, skin-side down. Keep the heat on medium-low and let the fat slowly melt away for 8 minutes or until the skin is thin and crispy. Turn over to brown the other side of the meat for a minute and then remove from the pan. Arrange in a baking dish and cook in the oven for 10 minutes, or 12 minutes if your duck breasts are on the large side. Remove from the oven, cover with foil, and rest for 10 minutes.
While the duck is resting, discard most of the duck fat from the frying pan, leaving just enough to coat the bottom. Add the shallots to the pan and season. Fry for 2 minutes, then add the vinegar and sugar and bring to the boil. After a minute, once syrupy, add the oranges to warm through. Remove from the heat. Serve the duck with the orange sauce and some chopped parsley, if you like.