Just to preface this, I don’t condone item duping, and have not confirmed this, nor do I plan to. I do believe that all information about games should be made publicly available and widely distributed. It looks like item duping was found before the game event launched. Only works on 360 right now, watch the video for the how-to after the jump.
I would expect gbx to fix this in the first patch, but no telling when that will come.