The Hexer is the unique bike you get in the beginning of the game. To duplicate this bike without using cheat codes, before the first proper mission where you steal the bikes with a truck, steal a car and drive to the meet-up, leaving the Hexer in the parking bay outside the safehouse. When you arrive at the meet-up with the other gang member, the car will disappear and be replaced by a second Hexer. Do the mission, pick the bike back up again, and park it next to the original outside the safehouse. You now have two Hexers.