The best way to ensure your duck breasts are perfectly cooked is to allow them to rest. While resting, the temperature will rise just a few degrees which makes it less likely to overcook them. So you basically take them out when they're not really done, and let them finish cooking in foil. The temperature won't rise more than 2-5 degrees even if you let them rest for longer. Resting duck - like any type of meat - also allows for the juices to settle back in, which gives you a juicier meat.