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Note this is not my video, I'm just sharing it to help you guys out.
With a text tut if you don't want to watch the video.

Step 1: Set your car that you want to store into your garage outside of your Garage/Apartment. (Make sure a friend watches your car)

Step 2: Go into your garage and select a car you're willing to get rid of for your NOS/Modded car.

Step 3: Once you hear the engine start up, pull out your phone and go to "Quick Job" and Click on "Random Job" it should say are you sure, don't accept it yet.

Step 4: While you are hovering over "Are you sure you want to join this job?" Press "A" and "RT" at the same time, and you should heard your garage door open while you join a job lobby.

Step 5: When you've loaded into a job lobby, leave that lobby. (B then A)

Step 6: Once you leave the lobby you should spawn in a grayish area, and then you should see your garage door. When you see your garage door quickly run out to the car you're duplicating, you should then glitch out and then drive it out your garage.

Step 7: If it says you still can't store this car, you get out of the car walk into your garage and then drive out in a random car. (don't worry you still keep the random car you drive out) After that, your car you want to duplicate should have vanished if it Vanished just drive back into your garage and it should be there.

Thanks guys, I know it's quite a long read I just wanted to make sure I could make it as detailed as possible so it could work first try for you all.
If any of you need further help post your gamertags and I'll try get back to you and help you out.