If an agent is created twice and one doesn't get assigned an id number, you can no longer make any updates to this agent. The agent will need to be manually removed.
The requester will need to have access to Adminer and a basic understanding of the database schema
Log into adminer of the affected ACD
Browse to table "User" and select
Filter the userId = 0
Find the user that is invalid and click the edit check box. Now click delete.