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I have a player in my game with an Aasimar Warlock with a Celestial Patron. At 6th level, Celestial Patron grants "... resistance to radiant damage, and when you cast a spell that deals radiant or fire damage, you add your Charisma modifier to one radiant or fire damage roll of that spell against one of its targets." Aasimar race also grants radiant damage resistance. Is there a rule (RAW) that tells what to do in cases like this?