Halve duck breasts and trim excess fat. Pat duck dry. Pierce skin of duck all over with a fork and score with a knife to allow fat to drain and help skin become crisp. Season duck with salt. Heat a large heavy skillet (preferably cast-iron) over high heat until very hot. Place duck breasts, skin-sides down, in skillet and immediately reduce heat to moderately low. Cook duck 20 minutes, or until skin is mahogany-colored and most of fat is rendered. Remove fat from skillet with a metal bulb baster or by very carefully pouring it off. Turn duck over and cook 2 minutes more. Transfer duck to a cutting board and let stand 5 minutes. (Duck will continue to cook as it stands until it reaches medium.)
While duck is standing, in a small saucepan boil demi-glace until reduced to about 3/4 cup and add vinegar, sugar, and peppercorns. Remove pan from heat and keep sauce warm, covered.
Diagonally cut duck breast halves and serve with sauce.
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