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Preheat oven to 350°F. Grease a 9x13 baking dish. Boil potatoes in salted water until tender, about 15 minutes. Drain well.
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Mash potatoes with sour cream, butter, and milk until smooth. Season with salt and pepper.
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Fold in 1 1/2 cups cheese, half the bacon, and most of the green onions. Spread into prepared baking dish.
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Top with remaining 1/2 cup cheese and bacon. Bake 25-30 minutes until heated through and cheese is melted.
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Garnish with remaining green onions. Let rest 5 minutes before serving.