Season duck pieces with salt, pepper, cumin, orange peel and 1 teaspoon crushed garlic. Place duck in a bowl and pour half of beer. Cover bowl and marinate duck for 1 hour.
Heat oil in a large saucepan and add duck pieces. Fry duck in hot oil, turning frequently, until browned. Add marinating liquid, remaining beer, orange juice and the necessary amount of stock or water. Bring to a boil and cook for approximately 2 hours until meat is very tender. Remove meat from saucepan and cool. Carefully remove bones from pieces, and keep warm.
Force cooking liquid through a strainer and return to saucepan. Bring to a boil and add potatoes, cut in medium size pieces. Cook until soft and tender. Heat oil in a skillet and add remaining garlic, onion and ají paste. Cook until onion is tender and stir in ground peanuts. Add this preparation to sauce in saucepan. Stir and correct seasoning.
Heat duck meat and serve it covered with the peanut sauce. Serve with cooked white rice.