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?
You have resistance to radiant damage (nothing more)
The rule on Damage and Vulnerability states:
Multiple instances of resistance or vulnerability that affect the same damage type count as only one instance. For example, if a creature has resistance to fire damage as well as resistance to all nonmagical damage, the damage of a nonmagical fire is reduced by half against the creature, not reduced by three-quarters.
You are receiving the same benefit twice, one from your character's race, and one from its class. Unfortunately, one of these is essentially "wasted" in that it provides no further benefit beyond what you already have from the first instance.