Update 3/12/2015This duplication glitch has been patched out. If you still want to do it or make the game super easy then delete everything and take your console offline. Install game (offline) and play for a couple hours so you have at least one of each item you want to dupe. Dupe everything you want and once your done, save and exit and reconnect your console online. You will then download the patch and you will still have all your items but you will not be able to dupe after that so make sure you make about 250 of each item you want. Still super easy but your going to have to do it at the start of a new game because saves are not interchangeable with different builds (patches).
I was messing around online and found this glitch. The glitch is for basically unlimited everything except weapons and ammo from my experience. This glitch kind of ruins the game if you do it early because you'll be so powerful.
*** Spoiler - click to reveal ***1. Use the blueprint and make 100 lockpicks (or more).
2. Go to any player stash(next to the bed usually in a safehouse). Select lockpicks and move them all over to your stash( then )
3. Now you should have 100+ lockpicks in your stash. Go to them and highlight, then press and at the same time.
4. Wait a moment and all menu screens will close (if you did it correctly). Now you should be looking at your stash again.
5. Open stash. Now both menu screens will be open together(stash screen and retrieve screen).
6. Scroll and highlight the item you want and press
Now if you had 100 lockpicks, you'll have 100 of whatever item you picked (and the lockpicks of course). You don't have to use lockpicks, you can use whatever item you have the most of (lockpicks are just cheap to build).
To level up, pick up an air drop and use the method above. Go to quartermaster and turn in. If you want money, then dupe something of high value and sell.
Crezzy posted the same solution a few days after me but with a video which may help some of you.