eBay returns this error when, using the “Enable duplicate title detection” feature,
an “unique identifier” previously used by the module is the same of the current item (you can see the duplicated item number as the “item ID” in the error message).
The unique identifier is a code that the module generates and sends to eBay based on the title of the item + the target eBay country / markteplace in order to prevent two items with the same title from being published in the same account
(prohibited and penalizing practice on eBay).
Therefore check that an item with the same title has not already been sent by the module and is actually on eBay.
If you intend to publish the product anyway, change the title slightly so that eBay will not detect it as duplicated.
If you wish, you can “reset” the history of the unique identifiers at your risk, so that new IDs are generated by changing the “unique ID” in the module configuration to a different and unused number.