Re-importing a file that contains products already in the store, with option values that match existing variants.
Treat import as create-or-update by Handle, and keep a stable Handle per product.
The free Shopify CSV checker names the exact line for each problem. The paid Rescue Kit adds 20 more error cases, a template in Shopify's column order, and the same checker as a script for your pipeline.
See the Rescue Kit →