Preheat oven to 350°F and grease a 9x13 inch baking dish.
Cook spaghetti according to package directions until al dente, drain and set aside.
In a large skillet, melt butter over medium heat and sauté onion, bell pepper, and garlic until softened, about 5 minutes.
In a large bowl, combine cream of chicken soup, diced tomatoes, sour cream, Velveeta, 1 cup of the cheddar cheese, salt, pepper, and paprika, stirring until well mixed.
Add the cooked spaghetti, shredded chicken, and sautéed vegetables to the cheese mixture and stir until everything is evenly coated.
Transfer mixture to the prepared baking dish and spread evenly.
Top with remaining 1 cup of cheddar cheese.
Bake for 30-35 minutes until bubbly and cheese is melted and golden on top.
Let stand 5 minutes before serving.