To duplicate shiv materials at any time, first pick up the upgraded melee weapon at the start of Bill's Town. (The weapon is found on the roof of the small building near the wooden plank just before you enter the town.) This weapon lets you craft upgraded melee weapons and the glitch can only be done AFTER you acquire this weapon. To duplicate shiv materials, simply equip an upgraded melee weapon and then swap it with another melee weapon that you find in the environment (bat, lead pipe, 2x4, axe or machete). If you then swap and re-acquire the upgraded melee weapon, your shiv materials will be replenished plus you will still have your upgraded melee weapon. This glitch is very helpful if you are running low on materials and require a shiv for combat or to open a locked shiv door.