The following error occurs when you are trying to add a network login which is already present in the database: Cannot insert duplicate key row in object tps_user_network_login with unique index 'index$tps_user_nw_login$nl' The statement has been terminated.
1. Create a new query in query designer based on System - User Network Logins. Pick any attributes you would like - I'd suggest Network login, User.Name and User.Title. Sort the query as you would prefer in order to locate the record quickly or set the criteria to prompt for network login perhaps.
2. You can now right-click on the required User Network Login and select delete. If you don't get the delete option please check your privileges or login as an administrator if you can.
You can now save and run this network login query any time that you need to remove one from the system.