You might want to display the same SKU under different categories or names (for better SEO). One example can be: new green T-shirt that will appear under the new collection & green shirts collection. That means that the green Shirt will have 2 different listings on Etsy using the same SKU. When a new order comes we will deduce inventory quantity from all of the other listings with the same SKU on your store. The same concept works when you manage duplicate SKUs on Shopify.