The update patched this but you can do this instead...
When you reach a checkpoint during any campaign quest in Solo mode, drop the item(s) on the ground you want to duplicate (you can drop all the items in your inventory). Then, kill yourself by using Deo-Bombs or Molotov's. When the respawn timer begins to countdown, pause the game, and reload your checkpoint. When you respawn, all the weapons you dropped will be back in your inventory and will also still be laying on the ground where you dropped them.
I found it here http://www.cheatcc.com/xbox360/deadi...eatscodes.html











