Remove giblets from cavities of ducklings; set aside.
Rub cavities with salt and pepper. Combine 3 stalks chopped celery, apple, and raisins; spoon into cavity of each duckling. Close the cavity of duckling with skewers.
Combine lemon juice and butter; set aside.
Place ducklings, breast side up, on a rack in roasting pan. Bake, uncovered, at 350° for 1 1/2 hours or until drumsticks and thighs move easily. Baste ducklings often with lemon-butter mixture during last 30 minutes of cooking time.
Combine onion, 1 stalk chopped celery, water, and giblets in a large saucepan; cover and cook over low heat until giblets are tender. Strain stock, and set aside. Discard vegetables. Chop giblets, and set aside.
Remove ducklings to serving platter. Degrease pan juices. Combine reserved stock and pan juices. Add flour; stir well. Add giblets; cook over medium heat until thickened. Serve gravy separately as a side dish.