Preheat oven to 350°F; grease a 9x13 inch baking dish.
Cook spaghetti according to package until al dente; drain and set aside.
Melt butter in large skillet over medium; sauté onion, bell pepper, and garlic about 5 minutes until softened.
Mix soup, tomatoes, sour cream, Velveeta, 1 cup cheddar, salt, pepper, and paprika in large bowl until combined.
Stir in spaghetti, chicken, and vegetables to coat evenly.
Spread evenly in dish.
Top with remaining 1 cup cheddar.
Bake 30-35 minutes until bubbly and cheese melted and golden on top.
Stand 5 minutes before serving.