Rinse the duck breasts in cold water and pat dry. Cut each in half to make four servings. Prick the skin in several places.
Add the other ingredients to a small saucepan, stirring to blend together.
Heat at a medium-low setting until the sauce begins to thicken and boil. Remove from the heat.
Cut four large squares of heavy-duty aluminum foil, and place a duck breast half in each.
Divide the sauce between the four breasts. Try to cover the breast in sauce.
Seal the foil as tightly as you can around each of the four duck breasts.
Place on the grill at a medium setting with the seal of the foil packet facing up. Grill for about 1 hour. They should be quite tender at this point.
Remove the packets from the grill, and place on a lipped dish to catch and preserve the liquid. Remove the breasts from the foil and grill skin side down for another 5 to 10 minutes to brown.
Remove from the heat when done and slice on a platter, drizzling the sauce over the meat.