Why am I getting standard cost exists error although the FBDI upload is successful?

You would be getting this error if records exist for the import combination of cost organization, cost book, item, valuation unit, and effective start date in the Manage Standard Costs page.

On the Manage Standard Cost Import Exceptions page, look for the impacted data to fetch the record. Click the Error link to view the exceptions for the import process. Delete the existing values from the Manage Standard Costs Import Exceptions page and then reload the item costs.