In Fable 3 for the Xbox 360, there is an easy way to duplicate pretty much any in game items by exploiting the coop gift function in the game.
Start a coop game, and have player 1 go up to player 2 and press LB to go to the gift room. You can give pretty much anything, and up to 100k gold pieces. Give them whatever items you want to duplicate, and then press start to exit.
Have player 2 press start to go into the Sanctuary. Have them go to the gifts and reject them, then press start again to leave the Sanctuary, and exit the coop game by pressing down on the d-pad.
Now player 1 presses start to go back into the Sanctuary and receives the rejected gifts, however when player 2 joins again and goes back to the Sanctuary, the gifts will still be there for them as well.
This is the best method in the game to duplicate legendary weapons, since they can be gifted.
Please note that this can and probably will be patched, so avoid updating the game if you plan to use this method of duplication.
