Can occur when used items (unique SKU) are to be listed under the same EAN.
Examples:
Item 123 exists in condition New (1 EAN)
Items 1234, 1235, 1236 also exist multiple times in condition Used (same EAN)
The solution is to make the ebay_item_title attribute unique for the affected items.
Examples:
Phone 256GB with Retina Display → Phone 256GB with Retina Display 1234
Phone 256GB with Retina Display → Phone 256GB with Retina Display 1235
Phone 256GB with Retina Display → Phone 256GB with Retina Display 1236