Pour the pumpkinseeds into a large (10-inch) ungreased skillet set over medium heat. When the first one pops, stir them nearly constantly until all have popped from their flat to a roundish shape. Remove a few to use as garnish. Scoop the rest into a blender jar.
Heat 2 tablespoons of the oil or lard in a very large (12-inch) skillet set over medium. When hot, add the chile pieces and toast them, pressing them down and turn them over, until their color lightens on the inside and they fill the kitchen with their toasty aroma. Remove to a medium bowl, cover with hot water and weight with a small plate to keep them submerged. Let rehydrate 20 to 30 minutes.
While the chiles are rehydrating, cook the onion and garlic in the same pan until richly colored and soft, about 7 minutes. Scrape into the blender jar with the pumpkinseeds. In a mortar, pulverize the black pepper, allspice, canela, cloves and thyme. Scrape into the blender jar and add the bread. Drain the chiles and add to the blender, along with 1 ½ cups of broth. Blend until as smooth as possible. If the texture is gritty, strain through a medium-mesh strainer.
Set the very large skillet over medium high heat and add the remaining oil or lard. When hot, add the chile-pumpkinseed puree. Cook, stirring nearly constantly, until reduced to the consistency of tomato paste, 7 or 8 minutes. Add another 1 ½ cups of broth and let simmer for at least 30 minutes to bring the flavors together.
While the sauce is simmering, heat a gas grill to between medium and medium-high (or build a charcoal fire and let burn until the charcoal is covered with white ash and the fire is medium hot). Brush the duck breasts with oil and sprinkle liberally with salt. Grill the duck breasts to between medium and medium-rare, about 3 or 4 minutes per side, depending on the heat of your grill.
Season the pipian with salt (usually about 1 teaspoon, depending on the saltiness of the broth) and a touch of sugar to bring on the dried chile flavor.
Serve the pipian with the duck breasts. Grilled nopales and steamed red-skin potatoes are wonderful accompaniments to this very traditionally flavored dish.