Save the game, then drop the items to be duplicated from the inventory of your current character. Press Start and choose change your character. Import your character from the previously saved game. The game will bring in your character with all items in inventory at the time of your save, and allow you to collect the duplicates that are on the ground. Alternatively, save the game and use a Recall Potion to return to the Elfsong Tavern. Drop all your expensive items, then import your saved player. Pick your items up and sell them. You can do this as many times as needed, have to save each time before you import again or you will lose the gold you have made.