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Crush strawberries and mangoes in a large saucepan.
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Stir in lemon juice and pectin until combined.
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Bring to a full rolling boil over medium-high heat.
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Add sugar and return to a rolling boil.
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Boil for 1 minute, stirring constantly.
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Remove from heat and skim foam.
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Ladle into sterilized jars, leaving 1/4 inch headspace.
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Wipe rims, apply lids, and process in water bath for 10 minutes.
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Cool completely and check seals.