Turn the ducks onto their breasts and bake for another 30 minutes. The last 30 minutes turn the ducks breast side up. Remove from the oven and allow to rest for 20 minutes. While the ducks are resting make the mango sauce. Peel and slice the mangoes and place in a heavy saucepan with the water; simmer until they are tender but still hold their shape. Add the Cassis and simmer for about 5 minutes. Carve the ducks and arrange on the platter. Serve the mango sauce with the duck.