Each individual product must have its own unique SKU. For example, if you sell a t-shirt that comes in multiple colors, each color must have its own unique SKU.
We recommend not using the "Duplicate" button on a product in Shopify, as this will disrupt any existing inventory you have against the original SKU. Instead, create a new product from your Shopify store's product page.
If you have multiple listings for a single SKU, you will see the following message on the Product Page in the Client portal.
"We found XX products with SKUs associated with multiple Shopify Products"
Click "See products with multiple listings" to see which SKUs need to be updated in your store.