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Put a bunch of ice in a large mixing bowl. Cover the ice with water. Set aside.
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Bring a medium pot of water to a boil. Add 1 pound peaches, and cook for 1 minute. Transfer to ice-water bath to cool. Remove peaches from water once you can handle them.
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Peel and halve peaches, and remove pits. Slice peaches into wedges, and transfer to a medium bowl. Toss with orange liqueur, sugar, and salt. Cover, and set in a warm spot until peaches give off juice, about 20 minutes.
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Puree peach mixture in a blender or a food processor until completely smooth. Immediately stir in tequila and lemon juice. (Peach puree with tequila can be refrigerated, covered, for up to 4 hours.) Transfer to a pitcher, and stir in peach nectar.
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Fill each of 8 tall glasses with ice. Divide margarita amongst the glasses. Garnish with peach slices, if desired.