This error means that you are using the same product id for another product on Walmart, and you can’t use this product id if you are trying to list it as a new product. Walmart doesn’t allow the use of identical product id for multiple products due to the duplicate listing creation.
Assign a unique product id to the new SKU you are trying to list and send product data update to the channel.
1- Click edit to the product.
2- Override the new product id and type for the SKU.
3- Select the product, send product data update to the channel and sync the channel.