Substitute an additional cup of seltzer for the rum if you prefer to turn this summer cocktail into a refreshing tropical fruit punch.
Combine sugar and 1/2 cup water in a small saucepan over high heat; bring to a boil. Cook until sugar dissolves, stirring occasionally. Remove from heat; transfer sugar mixture to a small bowl. Chill.
Combine sugar mixture, mango juice, and next 4 ingredients (through grenadine) in a large bowl; mix well. Stir in soda. Serve over ice.
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