When multiple sellers sell the same product on Amazon using the same UPC, Amazon .
To prevent the same product being sold multiple times Amazon uses product matching to find and eliminate duplicate listings
The creation of product detail pages is limited to products that do not already exist within Amazon's catalogue. Creating a product detail page for a product already in the Amazon catalogue is prohibited.
Understand the steps for creating a listing for a product already in Amazon's catalogue.
We verify the authenticity of product UPCs by checking the GS1 database. UPCs that do not match the information provided by GS1 are considered invalid by Amazon.
If the product that you are listing has been confirmed to have no known UPC, EAN, ISBN, ASIN, or JAN code and if you are listing a unique selection, you can request a brand- or SKU-level UPC exemption. Visit How to list products that do not have a GTIN for eligibility information.
You can check GTIN existence here https://gepir.gs1.org/index.php/search-by-gtin