Put the duck strips in a shallow dish. Make a marinade by mixing together the five-spice powder, soy sauce, sesame oil and 1 1/2 tablespoons of the orange juice. Pour the marinade over the duck, stir well to coat, then leave to marinate in a cool place for at least 20 minutes. (If time permits, marinate the duck in the refrigerator for 2–3 hours. Let it stand at room temperature for about 30 minutes before starting to cook.)
Meanwhile, thinly peel the zest from half of one of the oranges, then cut it into fine strips. Add the strips to a saucepan of cold water, bring to the boil, then drain through a strainer and set the strips aside. Cut away all the peel and white pith from the oranges using a sharp knife, then cut the flesh into segments.
Heat the sugar and vinegar in a small saucepan until the sugar has dissolved. Blend the cornflour with 1 tablespoon of the orange juice, then pour the remaining orange juice into the saucepan. Bring to the boil, then add the cornflour mixture, stirring all the time until the sauce has thickened and cleared. Add the fine strips of orange zest, then turn off the heat.
Remove the duck from the marinade with a slotted spoon. Heat a wok or large frying pan, add 1 tablespoon of the oil, then when hot add the duck and stir-fry for 2–3 minutes until tender. Remove and set aside.
Add the remaining oil to the wok, add the spring onions and snow peas and stir-fry for 1 minute. Add the bean sprouts and stir-fry for a further 2 minutes, or until cooked but still crisp.
Return the duck to the pan with the orange segments, sauce and any remaining marinade. Bring to the boil and let it bubble for about 30 seconds, or until everything is hot. Serve straight away.