This is a very easy and fast glitch to do.
What you will need:
1.one of your friends
2.a job invite ( from lester lemar etc. )
3.empty spot in your garage
4.any car in your garage
STEP ONE: have your friend outside your garage looking at your garage door. Then go into your garage

STEP TWO: go to your car then pull up your job invitation to where it says invitation on top of the phone then get in your car

STEP THREE: press accept and the acceleration at the same time so A and RT if your on xbox360
STEP FOUR: your friend should see you coming out of your garage with the car but you'll be in the job menu. then click confirm settings and then quit the job.

STEP FIVE: you should then spawn away from your garage with your car and the duped car will still be by your friend.

STEP SIX: drive to your garage and take the car your driving into it. Then come out and take the other one in. You will have two of the same car in your garage take the original one out.

STEP SEVEN: go to Los Santos customs and sell it then you get your money for the car and you have your duped car in your garage to keep and drive around.

NOTE: YOU CANT DO IT AGAIN WITH THE DUPED CAR BUT YOU CAN DO IT WITH OTHER CARS IN YOUR GARAGE THAT ARENT DUPED.

- PARALYZE