How important is the product code?
The product code (and related SKU code for multi-variant listings) is what Xpress Lister uses as the 'unique identifier' to know whether a product is a duplicate or a new product.
All product (and SKU code combinations) need to be unique. Duplicate products/skus will not be imported. Instead, they will be visible on the error report that can be exported, reviewed, fixed and reimported.