In a small saucepan, combine zest, juice, vinegar, honey, chili flakes, salt and pepper. Bring to a boil and reduce mixture by 50%, until syrupy. Place duck breast, fat side down, on a cold grill and turn it on. This will give the duck fat a chance to render somewhat. Be prepared for some flare-ups (have a spray bottle of water handy), due to the excess fat dripping from the duck. When the fat starts to crisp up, flip it over to allow the flesh side to cook. Cook to desired doneness and allow to rest on a cutting board for five minutes before cutting (this will help avoid losing too much moisture). Slice the duck thinly and drizzle the hot glaze over it. Serve immediately. Makes 4 servings.