Preheat oven to 350 degrees F (175 degrees C). Grease a 9×13-inch baking dish.
Bring a large pot of lightly salted water to a boil. Cook spaghetti in the boiling water until cooked through but firm to the bite, about 12 minutes; drain. Transfer to a large bowl.
Melt margarine in a skillet over medium heat. Cook and stir onion in melted margarine until soft and translucent, 5 to 7 minutes; add to the spaghetti along with chicken, evaporated milk, cream of chicken soup, 4 ounces shredded Cheddar cheese, celery salt, salt, and black pepper. Toss mixture to evenly mix; transfer to prepared dish. Sprinkle 4 ounces shredded Cheddar cheese over spaghetti mixture.
Bake in preheated oven until cheese is melted and mixture is bubbling, about 45 minutes.
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