Remove giblets and neck from duckling; reserve for other uses. Rinse duckling with cold water; pat dry. Prick fatty areas of duckling with a fork at intervals. (Do not prick breast.) Lift wingtips up and over back, tucking under duckling securely. Combine next 4 ingredients. Rub over surface and in cavity of duckling. Cover, and refrigerate overnight.
Brush bird with melted butter; sprinkle with 1/4 cup flour. Fold neck skin under, and place duckling, breast side up, on a rack in a roasting pan. Stuff cavity with celery leaves and onion. Close cavity with skewers. Place under broiler 2 minutes or until lightly browned.
Combine 1 1/2 cups water, wine, and catsup, stirring well. Pour over duckling in roasting pan. Cover with foil, and bake at 375° for 1 1/2 hours or until drumsticks and thighs are easy to move. Remove from oven; discard celery leaves and onion. Transfer duckling to serving platter; keep warm.
Skim fat from pan drippings. Discard fat, and pour pan drippings into a small saucepan; set aside. Combine 1 tablespoon flour and 3 tablespoons water, mixing to form a smooth paste; gradually add to pan drippings, stirring until smooth. Cook, stirring constantly, until thickened and bubbly. Stir in olives. Pour sauce over duckling. Garnish with pickled peaches, if desired.