Red Mail (5e Equipment)
From D&D Wiki
You can treat this magic armor either as a chain shirt or a full set of chain mail, depending on how you wear it. While you are attuned and wearing it, you reduce any bludgeoning, piercing, and slashing damage you take by ¾. This effectively acts as though you have damage resistance (halved damage) on top of a second damage resistance (halved again), even though this normally is not possible.