Easy Duplication Glitch
First of all you need two drop the weapon or gun on the ground then kill your self but don't respawn pause the game reloaded checkpoint then go we're you left the gun or weapon and it stays there (helpful for building weapon materials) you can do it as much as you want.
Duplicate Weaopns (NO GUNS)
1- Select your Weapon for duplication (no guns)
2- Throw your weapon the quickly go to your inventory, before it hits the ground, and go to the weapon you are throwing, and drop it. Then exit, (You will not see two weapons on the floor)
3- pick up your weapon and go to the inventory, and select the one that is equipped and put in your item list ( the list you hold R2 for )
4- Go back to your inventory, and there should be Two of the same weapon.
I used this on my most expensive weapon, like a wakizashi, or katana, ect. Then sell all of them EXCEPT FOR ONE! That's important. I have over 100, 000 dollars, so have fun. ;).