yeah i had a huuuuuge success rate last night after watching that japanese video (thanks whoever posted that). however i use the del perro heights apartment. im convinced that you can do this glitch at any apartment. you just need a few things:
2 apartments in the same building
a car you want to dupe in garage A and a space for the dupe to go in garage B
ideally, an already existing situation in the built environment that allows you to park, have your doors blocked so that you warp in and out of your car. (for me, at del perro, this is inbetween the two dumpsters on the other side of the parking lot adjacent to the garage, roughly 200 feet away) (note: i have personally found this method to be WAY more reliable than the previous methods, ie parking the duper/original panto out of range of the garage door, this will be further explained)
now, you follow these steps to create the dupe, and continue duping. (i do this in an invite only, solo, but i guess you can do it with friends, no testimony on the buddy version)
pull the duper/original out of the garage
get your car into the warp in-out position, blocking both doors, assure that you can warp in/out by trying it
get an NPC car (the car type does NOT matter) and drive it near your duper/original
get out take a few steps and face the NPC car. select the on-call (middle left option) mission on your phone and start up a random on call mission alone (you will see a timer start in bottom right for on call)
go to quick mission (middle, middle option) on your phone and select a mission. before you fully accept this mission, you must be on your way into your car. (much speculation on timing here, i tried fully before touching the door, ive accepted after the doors been opened, both with great success. im not sure ive tried after ive fully sat down, but who cares, its not hard to time it for the other two options)
drive your npc car over to your garage and attempt to enter. you will get an option for which garage you would like the car in. hover over the option with the empty space and wait.
the mission will load up. quit the mission (circle/B), confirm that you would like to quit the mission (x/a), then actually make the selection of which garage you want the car in (if you were hovering over it before you can just hit x/a) (also much speculation here. ive effed up multiple times and accidentally stayed on the mission screen for up to 5-8 seconds before quitting)(even more speculation about how fast to hit the final X, X... i havent definitively found that pressing too fast or too slow changes anything, as ive pressed seemingly too slow and seemingly too fast, and still spawned outside my garage)
hopefully you will spawn outside your garage, near where you accepted the mission. This is why parking the NPC car further away seems to work better. after the mission quits you spawn near where you accepted the job.
press triangle/Y to enter your car, if you spawned close enough, you shouldnt need to see your car or anything, just hit triangle/Y ONCE. if you warped into your car, dont move. it will either have you sit there until you spawn in your garage with the duplicate, you will see your dupe warp to the position of the NPC car and drive into the garage, or you will see yourself driving into a seemingly empty structure. ALL ARE WINS. note: if you did not spawn in your car after hitting triangle, try reconfiguring where you start with the NPC car, move it closer to the original, and continue from step 4.
walk out of your garage, walk down the street to where your car is on the minimap, and hop in. this is your original. if you wish to continue duping, drive it back into the warp position, grab another NPC car, rinse and repeat.
THAT WAS HOW TO DUPLICATE THE VEHICLE. TO SELL THEM FASTER THAN 1 EVERY 48 MINUTES (IRL), YOU MUST DO THE INSURANCE GLITCH TO BYPASS THE WAIT.
noteworthy mentions:
i have spawned directly into my garage after quitting the mission. it seems there is a way to tell right before it happens. if you see the black mission screen FADE out as you quit, it will spawn you in your garage. if you see a harsh transition (no fade whatsoever) after quitting the black mission screen, you did it correctly and will spawn outside your garage. i can only offer the same speculation that you all have on this: maybe you quit too fast. it seems to me, that you can not quit too slow, but it may reduce the time you have to realize you are outside and then to warp into the original.
if you see a 'bricked' (un-enterable) NPC car blocking part of your garage door, you can still get this glitch to work (i had a brick there for 3-4 of my dupes last night). you do nothing differently. however it seems that the above mentioned fade out has more potential to happen. but, i had not had it happen 2x in a row, even if the brick is still there. if you want to ensure the brick is not there, just drive around the block. it should despawn.
sorry for the super long post, but hopefully this will eliminate questions and pointless, "i cant do it, dont try, doesnt work" posts. If you have a question for me, i will try to answer it and add it to this post ASAP.