I think that the issue is that even if you manually import a CSV file with a duplicate Key within the imported data, it will fail.
Yes, if the Key exists it will update and if the Key does not exists it will add. But if you try to import data with duplicates
within the import set in the Key it will fail, so I think that the ACTION / WEBHOOK is just an automated version of the manual process.
So, I don't know how to do that without a script as they would need to be added one by one after checking if they exists or else added one by one one record at a time so that there is no way that the import record set of 1 record can have duplicates within the set.