"... One night after a while playing co-op, I decided to screw around solo. In co op mode, the action doesn’t stop when you pause or bring up a menu. In solo mode it does. Apparently, due to this item, duplication is extremely easy in solo mode, but it only works with melee weapons. You can exploit this for easy money or for an arsenal of 2-7 of the same powerful weapon. It is very easy to do." - JPS
the video is now up! You can check it out if you're having problems following the steps.