When you reach a point in the game where you can pick the Knights you are taking with you from the Round Table before a mission, choose the person whose item you wish to duplicate and the person you wish to have the item for. The person that has the item equipped must then go into battle and be killed, allowing you to equip the item(s) they had on another Knight. After that, complete the mission and allow the game to auto-save. At the start of the next mission, choose the same people as before. You will notice the Knight you allowed to die has the same equipment they had on at the start of the mission, and the Knight you gave their items has the item as well. This will allow you to have duplicated items as long as you do not remove the item from one of the Knight's inventory. This can also be a good way to save money in the Treasury section of the game, when some of the better equipment is a bit on the expensive side.