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Make a simple syrup by combining water and sugar in a saucepan, boil, reduce heat, and remove to a bowl to cool
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Blend watermelon and tequila in blender until smooth
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Drain through a fine mesh strainer into a bowl
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Combine watermelon/tequila mixture in a pitcher, add simple syrup and lime juice
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Stir to combine
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Transfer about 3/4 cup of watermelon mixture to cocktail shaker with ice, shake until cold, pour into margarita glass and add a splash of spumante and a splash of club soda
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Garnish with a lime slice and salt if desired