If you get the error "The same Merchant SKU was found among 3rd Party Listings. Merchant SKU must be unique for each Amazon item" during the listing of a new product, it means that you already have an active Item with the same SKU on Amazon, and it was imported into 3rd Party Listing.
If the products are identical, you can manage the 3rd Party Item via M2E Pro instead of listing a duplicate. Please complete the following steps:
1. Apply the action Stop on Channel / Remove from Listing to the duplicate product in M2E Pro Listing.