you need a clubhouse, a lowrider you wanna dupe (i use the sultan, near 800k per dupe), empty spaces in reg garages and a faggio at the clubhouse.
1) drive lowrider to clubhouse... blow it up.
2) walk in clubhouse, sit on faggio... call Mors and make a claim on your lowrider.
3) while still sitting on faggio, check map to make sure your car's icon is at Mors.
4) right on dpad, taking faggio into mod shop.
5) respray faggio, leave mod shop, GET OFF BIKE.
6) walk outside, STEAL CAR, drive to Mors, get in lowrider. DO NOT CALL FOR YOUR MECHANIC TO DELIVER!
7) drive into lsc or bennys, change anything. i change the plate for 200. and drive out.
the dupe is complete... back to step 1. no need to drive to a garage and store it, etc... does that automatically.
best bet is to wait for next update... usually right after an update people find new ways to dupe. they're typically easy, can usually be used on any vehicles, no time limits for selling, etc.