The Duke O'Death is a special vehicle that must be unlocked first by completing the 'Duel' random event in GTA 5 single player. That random event is only available on the next-generation PS4, Xbox One and PC versions of the game, so unfortunately if you have the older generation Xbox 360 and PS3 versions of GTA 5 you won't be able to get it.
https://www.youtube.com/embed/gdiiCiINzow
That said though, the vehicle looks awesome and is sure to strike fear into the hearts of any pedestrian in the game as you hurtle towards them, full speed. So enjoy it!
