Preheat the oven to 220 C, 200 Fan, Gas Mark 8. Slash the skin of the ducks diagonally, about 4 times on each breast. Sprinkle over the soy sauce and rub on to the skin with your hand.
Arrange skin side up on a rack over a baking tray. Cook for about 20 minutes but check after 15- the skin should be crisp, the flesh pinkly tender.
Meanwhile peel the oranges removing as much pith as possible. Slice the oranges crosswise and then chop each slice into small chunks.
Chop the red onion finely. Cut along the chilli lengthways, deseed and chop very finely.
Mix together the onion, orange, chilli, mint, coriander and a little salt. Squeeze over the lime gradually, tasting until to your taste. Mix well and set aside until needed.
When the duck is cooked rest for 5 minutes before slicing diagonally and serving.