I watched some youtube vids before and it shows Duke O Death in single player mode resists one RPG (and it leaks fuel, catches fire and explodes eventually). However when I got my Duke O Death in Online Mode and I tried to shot a RPG on it, it exploded instantly.
It says the windshield of Duke O Death was completely non bullet proof although all other windows including the tiny one in front of the copilot seat are well protected. Is it buffed in Online Mode? I hope so otherwise my favorite car in game is only good at its looking.