Preheat the oven to 450 degrees. Break up the duck carcasses and place them with the necks, wings, and feet in a roasting pan. Roast until browned, about 30 minutes. Using a slotted spoon, transfer the duck parts to a stockpot, add enough cold water to cover, and heat to boiling. Skim the scum from the surface. Reduce the heat and simmer uncovered.
Meanwhile, pour off most of the fat in the roasting pan. Add the onions, leeks, and garlic to the pan and roast until browned, about 30 minutes. Add the vegetables to the pot. Pour ¼ to ½ cup water into the roasting pan, heat, scraping up the browned bits on the bottom, and add it to the pot. Add the peppercorns and herbs. Simmer uncovered 4 to 6 hours.
Strain the stock and chill, then remove the fat from the surface. Boil until reduced by about half. This stock can be stored in the refrigerator up to 1 week, or in the freezer up to 6 months.