According to the "Resistance and Vulnerability" section in the introduction to Xanathar's Guide to Everything (p. 5):

Here’s the order that you apply modifiers to damage: (1) any relevant damage immunity, (2) any addition or subtraction to the damage, (3) one relevant damage resistance, and (4) one relevant damage vulnerability.

Even if multiple sources give you resistance to a type of damage you’re taking, you can apply resistance to it only once. The same is true of vulnerability.

What counts as "(2) any addition or subtraction to the damage"? It seems like these additions or subtractions will bypass damage immunity.