The Imponte Duke O'Death will appear as a Random Event after you complete the first Tonya Strangers and Freaks mission Pulling Favours with Franklin - found opposite his aunt's house at the beginning of the game.
The Random Event appears by the cafe on the south road across Sandy Shores, just below the airstrip.
After stealing it and driving off you get chased by several articulated trucks in scenes reminiscent of the film Duel, as well as a number of gunmen in smaller vehicles.
Take them out or escape to unlock the Duke O'Death, which can be stored in your garage and also respawns in the following two locations:
By the oil derricks west of the airstrip in Grand Senora Desert.
Inside a covered area in the southwest corner of the scrapyard in El Burro Heights.
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