If it is actually blocking you and not letting you save an employee with the same SSN, then that's very bad because there are more than a couple of reasons where an employer needs to do this in QuickBooks, at least one is legal, when an employee changes legal status so that they are nos subject to SS and MediCare when they no longer were.

Is QuickBooks actually stopping you, or can yo proceed through the message. If you cannot, then some uninformed person at Intuit thought this was a good change - but it's not.