When creating a user in Service Catalog, the error:
"Error - User with this a user id already exists. Choose a different user id"
appears but the user id cannot be found in the Service Catalog UI.
A user with the same user ID previously existed in Service Catalog, but has been deleted at some point. This old user account was preventing the creation of the new user. When a user is deleted in Service Catalog, the record isn't physically deleted from the database. Instead, a flag called 'inactive' is set to 1, rather than 0. This means that the database record still exists, and when you try to create a user with the same user ID, it fails as the user ID must be unique.
Using an appropriate database query tool, locate the database record in the ca_contact table in the mdb for the old deleted user. Change the "inactive" field to 0, and then this user will re-appear in the Service Catalog GUI. It can then be changed to reflect the amended user details for the user which needs to be created. CA do not recommend deleting the ca_contact record because there could be existing information in Service Catalog which depends on this record.