Hmm..to me, I would list additional items as extra quantity, as long as its the same exact item. So, if you're selling a set of 10 red beads, and you have another set of 10 to sell, I would change the quantity available to 2. This way, if one sells, you don't have to relist it right away, because there will still be one available.
As a buyer, I would prefer not to see duplicates of the same item more than once in a shop, unless something is different about it (quantity, size, etc..)