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Preheat oven to 325.
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Grease a 9x5-loaf pan with butter. Sprinkle with flour and set aside.
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In your mixer's bowl cream together butter and sugar; beat until smooth and creamy.
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Add egg and vanilla, and continue to beat until well incorporated.
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Using a rubber spatula, gently fold in mashed bananas and 1/2 cup flour. Set aside.
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In a separate bowl, with a whisk, combine remaining flour, cinnamon, baking soda, baking powder, and salt.
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Fold flour mixture into banana mixture until thoroughly combined.
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Gently fold in blueberries.
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Pour batter into prepared pan.
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Bake for 45 to 50 minutes, or until a toothpick inserted in the center comes out clean.
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Remove from oven and let cool.
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Invert onto a rack.
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Cut and serve.