Two or more products share the same GTIN, multipack, and condition combination, so Google can't tell them apart.
Product identifiers (GTIN, MPN, brand) anchor Google's matching of your product to its catalog. Issues here typically come from non-numeric characters in GTIN, mismatched UPC vs EAN length, brand-GTIN ownership conflicts in GS1's database, or duplicate GTINs across multiple SKUs.