Equip any set of arrows, go into any area of the game (preferably an enclosed area, like a fighter's guild basement), take out any bow, and hold R to keep an arrow in your bow. Now, you have to be a journeyman(?) of acrobatics, but if you can combat roll, that's what you need to do this trick. Hold L, press Y, and move in any direction to roll. You will hear the arrow fly, but it will not decrease in your inventory. Go forth and pick up your dupe arrow. Repeat as many times as you like.