This question already has an answer here:
The Rune Priest in my group grants DR4 to anybody next to him. Not a problem. But then, my players heard me say: "He swings his axe against the paladin of Khalmyr and: 15 damage plus 8 cold damage."
That was all it took for the Rune Priest (who was feeling useless until the cleric stopped playing with us) to say: "Sooo... Does the DR I grant count twice? 'Cause it is 15 from the axe and 8 from the cold, right?"
And I seriously didn't know. If does make sense, though. Because if they had used a potion of resistance, it would only count against the extra damage.
So... What is the correct way?