Superannuation - new employees

I have two new employee's that started the same month, both are teenagers and never had superannuation accounts.  Previously when I have had this situation, I have been told to type "UNKNOWN" into my default superannuation account where their account number is entered.  Now when I do to lodge these two employee's, it's saying I have used this number "UNKNOWN" from a previous employee (Note: the employee's that is came up is from month's ago and I have since entered his correct account number into his card file) 1) is this doing this because I am trying to enter to new staff member's and 2) when I click on the "X" to find out why it will no process its saying the old staff member that has since received an account number and it has been amended on MYOB.