Go to player stash in the closest safehouse and select an item thats ×2 or more (not weapons). Tap X and O at the same time and the item screen should remain. Wait until it disappears and then you open the bag again. Scroll to the item you want to duplicate (not weapons) and tap triangle. The item will now be ×2 or more in the bag. BEWARE: if you have an item thats ×10 and do this glitch with an x2 you will have 2, thereby loosing 8 in the process. The recommended item to use is lockpicks because you can get huge amounts from very little resources.