Easiest method seeing as most servers have attempted to remove ways to do the others.
1. Buy or find a car that can hold atleast 1 bag
2. Place all items you want duped into the bag you are currently wearing (I don't suggest duping weapons)
3. Stand outside you vehicle and access the cars gear
4. Get ready to drop your bag into the car
5. Now turn your internet off any way you can, either by wi-fi or ethernet cable
6. Drop the bag into the car
7. Back away from the car and abort from the game (some servers don't allow you to log out right next to a car as an anti dupe counter measure)
8. When you are back into the lobby menu reconnect your internet
9. Once your internet connection has been re-established enter back into the game
10. Once you are back in the game you will have a bag on your back, and another in your car, both with your items.
11. Enjoy
You should really search the forums for an answer before posting your own thread, there are plenty of duping methods on these forums.