Whisk together the fish sauce, thick soy sauce, honey, and five-spice powder in a large dish. Carefully loosen the skin of the duck by gently running your fingers between the skin and the meat, trying not to tear the skin in the process. Place the duck in the marinade and rub the marinade over it, inside and out and between the flesh and the skin. Drain and refrigerate the duck, uncovered turning occasionally, for 24 hours.
Preheat the oven to 425 degrees. Place the ginger and scallions in the cavity of the duck, then truss it with kitchen string. With a metal skewer, poke the skin of the duck a few times on the breast and legs. Place the duck, breast side down, on a rack and roast for 45 minutes, basting every 15 minutes. Turn the duck breast side up, and continue roasting until juices run clear, about 45 minutes.