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For basil simple syrup:
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In a small saucepan over medium heat combine sugar, water and basil; bring to a simmer.
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Remove from heat, set aside at room temperature to cool, at least 30 minutes.
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Discard basil, cover simple syrup, and refrigerate.
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Can be made up to 1 week ahead. Store in refrigerator in an airtight container.
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Rub rims of glasses with 1 lime wedge.
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Dip rims of glasses in salt mixture.
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Place glasses into freezer to chill.
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In a pitcher stir together watermelon juice, tequila, lime juice, cointreau and half of basil syrup. Taste; add more syrup if needed.
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Remove glasses from freezer and serve margarita mixture over ice.
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Garnish with lime slices