Preheat oven to 500 degrees. Place sour-cherry preserves in a food processor, and puree. Transfer puree to a small bowl, and set aside. Place peppercorns on a clean cutting board. Using the flat side of a heavy knife, press down on peppercorns until they are coarsely crushed, and set aside.
Cut both duck breasts in half, and trim away any excess fat, leaving the skin attached to the breasts. Season both sides of duck breasts with salt and pepper. Spread 1 tablespoon pureed preserves on the flesh side of each breast; sprinkle with half of the crushed peppercorns.
Place duck breasts, flesh side down, in a medium oven-proof skillet. Spread 1 tablespoon pureed preserves on skin side of each breast; sprinkle with remaining crushed peppercorns. Place skillet in oven; roast until medium rare, 10 to 12 minutes. Remove from the oven; set aside in a warm place.
Melt butter in large skillet over medium-high heat. Add cherries, plums, sugar, and thyme. Season with salt and pepper. Cook until sugar dissolves, 1 to 2 minutes. Add port. Strike a match, and carefully ignite port; cook until flame dies out. Add vinegar, and cook until juices thicken, 10 to 15 minutes. Remove from heat.
Transfer duck to a serving dish, and pour fruit sauce over top. Serve.