Rinse the duck under cold water and pat dry with kitchen paper. Rub the inside and outside with salt and pepper.
Dice one mango and cut the remaining three mangoes into slices and set aside.
Place the diced mango into a bowl and mix with ginger, cardamom and cinnamon. Stuff the duck with the mixture and secure the cavity closed with skewers or kitchen string.
Melt butter in a casserole dish and sear the duck on all sides. Cover and cook the duck over low heat. You should expect a cooking time of about 20 minutes for every 500g. Baste duck occasionally in its juices.
Take duck out of the casserole dish, let it rest for a few moments and then carve.
Skim the fat from the casserole juices and season with salt and pepper to taste. Serve with the carved duck and reserved mango strips.
Very ripe mangoes work best for the stuffing. If you cannot find ripe mangoes in the supermarket, place unripe mangoes in a airtight container, together with an apple. This will speed up the ripening process.