In this supplement for Rise of Tiamat, on page 4, the properties of the "dragon mask" magic items are described, including this property (bold emphasis mine:
Damage Absorption. You have resistance against the mask’s damage type. If you already have resistance to that damage type from another source, you instead have immunity to that damage type. If you already have immunity to that damage type from another source, whenever you are subjected to damage of that type, you take none of that damage and regain a number of hit points equal to half the damage dealt of that type.
Off the top of my head, I cannot think of any other feature, be it class feature, racial feature or magic item, that allows multiple forms of resistance to stack and become immunity to either a damage type or a condition. Is this a unique feature of the dragon masks (I’m not so interested in the absorption part, just the stacking resistances), or are there are features or magic items RAW that allow stacked resistances to become immunity?