With a sharp knife, trim the skin and excess fat from the duck breasts, leaving only a thin layer of fat. Make criss-cross incisions on the remaining fat of each duck breast, without cutting into the meat.
In a glass dish or a large sealable plastic bag, combine the vinegar, oil, garlic, sugar and salt. Add the meat and thoroughly coat with the marinade. Cover the dish or seal the bag and refrigerate for 12 to 24 hours.
Preheat one side of the grill, setting the burner to high.
Oil the grate on the unheated section of the grill.
Place the duck breasts, fat side down, on the unheated side of the grill. Close the lid. Cook for about 15 minutes. Transfer the breasts onto the heated section of the grill and cook for 3 to 4 minutes on each side, or until medium rare. Season with pepper. Set aside on a plate and tent with aluminum foil. Let rest for 5 minutes before serving.
Recipes with the "Healthy Pick" stamp have been evaluated by a registered dietitian member of the Ordre professionnel des diététistes du Québec.