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Duping is a glitch in the game people use to speedrun, basically triggering the same mission multiple times. One example where it's often applied is, I think, Taking Out The Laundry, or whichever one Toni gives you where you're meant to blow up the three Triad vans.
If you trigger it twice, then six vans spawn. If you then kill yourself, you fail one of the instances of the mission, and all the vans disappear. But then the other instance of the mission detects that there are no vans remaining, and thus thinks you passed the mission.
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