To duplicate crafting materials and weapons, enter another player's game online or through system link. Then, drop the items and weapons you want to duplicate. Once you have finished dropping the items, do not leave your inventory. Instead, press the Xbox Guide button, and choose to go to the dashboard. Next, go back into the game. You will have all of the items and weapons you dropped back in your inventory, plus all of the items and weapons you dropped will still be on the ground. Repeat this process as many times as desired. Sell the items to get as much gold as desired. Note: Sometimes the game will auto save if you drop too many items. Thus, pay attention to the top right corner of the screen to see if it auto saves. If it does, simply pick up your items and complete the mission to get the game to auto save with your inventory intact. Then, try the exploit again.