Rinse the duck and place on rack in baking tray.
Combine 1/2 of the water, 1 tbsp chopped chillis, vinegar, soy sauce, and hoisin sauce, and the 1/4 tsp 5-spice powder. Pour mixture over the duck.
Roast the duck uncovered in preheated 220C oven, basting and turning frequently until light brown, about 20 minutes. Reduce heat to 180C and roast the duck, basting and turning frequently, 1 hour longer.
Remove the duck from oven. Cool completely. Cut the duck in half and remove and discard the backbone. Cut into small serving-size pieces.
Remove top leaves and all skin from the pineapple. Cut into quarters and discard the core, slice into chunks.
Slice the onion and heat the oil in a wok over high heat. Stir-fry ginger and garlic in the oil for 1 minute. Add the duck and stir-fry until the duck is hot throughout, 3 to 4 minutes.
Combine remaining water, 1 tbsp chilies, 1 1/2 tbsp each vinegar and soy sauce, 1 tbsp hoisin sauce, and the cornflour.
Pour the mixture over the duck. Cook and stir until liquid boils. Add the pineapple and onions. Cook and stir until the pineapple is hot, about 2 minutes longer.
Serve with sticky rice and garnish with green onion curls.