Allright, so it's not actually duplicating, but kind of. What happens is that you let others take the control of your own mammoth / motorcycle so you can dismount / logout while the player keeps controling your mammoth, riding around alone.
What you do is:
Get on your mammoth, dump an NPC so you can bring a mate onto one of the 2 extra slots. And then at exactly the same time, you remove your mammoth icon on the top left of your screen while your mate clicks your mammoth to get a ride. So this is what happens now, you jump off the mammoth and your mate takes your slot in the main-seat and the mount is now his until he dismounts or you try to use your own mammoth.
HD share - Mammoth Trick
So to the "duplicate" part then. What I discovered is that if you relog, you will be able to use your own mammoth, even though your mate is still using your old one.
This means anyone from level 1 to 80 can ride around on your mammoths, have fun while it's lasting.
*edit* vehicle, not mount