At some point, someone deleted the employee while the Social Security Number was still entered in the SSN field on the Employee Detail screen.
When am employee name is deleted it does not actually go away. The name is just buried deeper in the database. When an employee leaves your organization you should not delete his/her name. Instead, you should change the Employment Status to Inactive or Terminated.
If you prefer not to see Inactive or Terminated employees, click View > Employee Display Filter and uncheck Inactive and Terminated an then click OK.
Deleting employee names should be rare but when you do,
you must first delete the SSN and the ID from the Employee Detail screen. If you don't do this you will get a Duplicate SSN error or Duplicate ID error when you try to re-enter the employee later.
You may move the SSN and the ID into the Comments field at the bottom of the screen.
Select the employee and Click File > Delete Employee as shown.
If you need to completely remove deleted employees, please click here.
To resolve this, you will need to view Deleted Employees, locate the one with the duplicate SSN and remove the SSN.
Viewing Deleted Employees:
Click Employee Detail and search through the names for the employee with the duplicate SSN and delete the SSN.
View all Active Employees Again: