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Bring a pot of salted water to a boil. Cook penne according to package directions.
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Melt the butter over medium heat. Add the flour and stir for 1 minute. Add the half-and-half and stir until thickened, about 6 minutes. Remove from the heat and add the Parmesan, salt and pepper. Cover.
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Pound the chicken to 1/2-inch thickness. Rub Cajun seasoning and cayenne evenly over the chicken.
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Heat the oil in a nonstick skillet over medium-high heat. Add the chicken and cook, turning once, until no longer pink in the center, about 6-8 minutes. Slice the chicken.
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In a bowl, toss the pasta with the sauce. Top with chicken and tomatoes and serve.