Combine first 5 ingredients in a bowl, stirring with a whisk. Add duck; cover and marinate in refrigerator for 20 minutes.
Prepare grill to medium heat.
Remove duck from marinade, reserving marinade. Bring marinade to a boil in a small saucepan over medium-high heat, and boil 1 minute. Remove from heat.
Place duck on grill rack coated with cooking spray. Grill 6 minutes on each side or until a thermometer inserted into thickest portion registers 170°, basting occasionally with reserved marinade.