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HEAT grill to medium heat.
MIX meat and 2 Tbsp. ketchup just until blended; shape into 4 (1/2-inch-thick) patties.
GRILL 5 to 6 min. on each side or until done (160°F). Top with Singles; grill 1 min. or until melted.
SERVE in buns topped with remaining ketchup and mustard.