In a mob grinder I often see mobs immediately picking up armour pieces dropped by the preceding killed mob. While witnessing this, I saw a fully gold-armoured zombie pick up golden armour drops from the previous mob, despite already wearing the same thing.

They don't seem to drop their own armour when doing this, either; they just 'absorb' the new armour piece on top of their own, and drop it as item when they're killed, like mobs normally do with picked up armour.

I specifically checked that they weren't holding the armour in their hands as item.