Step one- go to your garage.

Step two- get into the vehicle that you want to dupe. (Do not go out of the garage. just sit in it.)

Step three- request a mission

Step four- accept the mission.

Step five- once the mission starts it should glitch out a little and spawn you outside of your garage somewhere.

Step six- drive back to your garage and enter it with your car.

Step seven- walk out of your garage and cancel the mission.

Once you spawn back into another online game your duplicated car should be waiting for you in your garage in a different parking spot

let me know if this works for you guys. as it has worked for me .