If you have multiples of the same item, you can sell them all from a single listing by setting the quantity as more than 1.
How it works
When your listing's quantity is set as more than 1, the listing won't be marked as sold automatically after it's purchased. It stays visible in your shop, and more Buyers can keep buying the item.
The listing stays available until you manually mark it as sold. When you run out of stock, set the item as sold so no further purchases can be made.