Cut the duck breasts into large squares (perhaps 8 pieces) and cover the meat with water in a cooking pot. Add the rice wine or sherry, onion, garlic and peppercorns and bring to a simmer. Simmer covered untilt the meat is tenderly cooked. Turn off the heat and let the meat cool off completely in the broth. This will take hours.
Remove the duck meat and shred it apart by using two forks on a cutting board or plate.
Serve the warm(ed) tortillas with the duck meat,
salsa cruda and pineapple, or serve the tacos pre-made.