Shopify: Duplicate handle

Two rows claim to be the same product but Shopify cannot tell them apart. A repeated Handle is legal only when the extra rows are variants of the first one.

How to fix it

1. Find the repeated Handle in your file.
2. If the rows really are the same product, merge them into one product block: first row carries Title, later rows carry only option values, SKU, price and stock.
3. If they are different products, give one a new unique Handle (lowercase, hyphens only).
4. Re-import.

How to avoid it next time

Generate handles from the product title once, at creation, and never regenerate them later.