To prepare duck, place breast halves between 2 sheets of heavy-duty plastic wrap; pound each piece to 1/4-inch thickness with a meat mallet or small heavy skillet.
Combine peppercorns, coriander seeds, salt, and garlic. Rub peppercorn mixture evenly over both sides of duck; let stand 15 minutes.
Heat 1 teaspoon oil in a large nonstick skillet over medium-high heat. Add half of duck; cook for 1 minute on each side or until desired degree of doneness. Remove duck from pan; keep warm. Repeat procedure with remaining 1 teaspoon oil and remaining duck.
To prepare sauce, combine port, sugar, and vinegar in a saucepan. Bring to a boil over medium-high heat; cook until reduced to 1/2 cup (about 10 minutes). Remove from heat. Add half-and-half and butter; stir until butter melts. Cut duck breasts crosswise into thin slices. Drizzle with sauce and sprinkle with parsley, if desired.