Each employee can only have one E2N ID and each E2N ID should only occur once.
In the administration you will find a clear list of all assigned E2N IDs.
You can find the assigned E2N ID via the search by entering the name and vice versa.
Click on the blue pencil next to the number to edit or delete the employee's E2N ID.
Here you can read more about the E2N IDs.