Pour olive oil into a 10- to 12-inch nonstick frying pan over medium-high heat. Add duck breasts and cook, turning once, until well browned on the outside and barely pink in center of thickest part (cut to test), 12 to 15 minutes total. With tongs, transfer duck to a rimmed plate and cover with foil.
Add onion to pan and stir often until limp, about 5 minutes. Add orange juice, broth, marmalade, lemon juice, vinegar, and orange slices; boil, stirring occasionally, until mixture has thickened and is reduced by half, about 15 minutes.
Meanwhile, in a 1- to 2-quart pan over high heat, bring 1 1/4 cups water to a boil. Stir in couscous, cover, and remove from heat. Let stand 5 minutes.
Pull duck into 2-inch-long shreds, or cut into 2-inch pieces or 1/4-inch-thick slices. Add to orange sauce along with any accumulated juices and the olives; stir until mixture is hot. Add salt and pepper to taste.
Gently fluff couscous with a fork. Spoon onto plates and top equally with duck and sauce.