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Preheat oven to 375 degrees.
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Cook macaroni in 5 quarts boiling water for 8 minutes. Drain macaroni.
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In medium saucepan, melt butter.
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Stir in flour, mustard, and salt. Gradually stir in milk. Cook and stir until mixture thickens slightly.
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Remove from heat.
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Add 1 1/2 cups cheese to milk mixture and stir until melted.
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Stir in macaroni.
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Turn into greased 2 quart baking dish.
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Top with remaining cheese.
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Bake 20-25 minutes or until bubbly.
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Refrigerate leftovers.