Gaming: A Pet Peeve

Don't make your player buy a duplicate of an item if they already own it. If the player owns an object and it is in storage allow the player to pull it out and place it in the game to fulfill all relevant portions of a quest, not just one portion. Once the item is placed in the game update the quest so that the requirement to buy one from the shop is fulfilled. If it's not the player has no choice but to waste gold buying a duplicate to fulfill it. If the intent of requiring the player to buy a duplicate of an object is as a money sink then mark the original object as broken. It will introduce a new object state in the game but this could be used to an advantage to soak up all the extra gold floating around.