Ability to "Lock" Creation of Duplicate Case Identifiers

Customer indicated the warning that a case id is already in use is not enough to deter users from creating duplicate case ids. In turn this causes major accounting export issues. They want to allow their locations to manually enter case ids, but to also have the ability to "lock" entering a duplicate case id. Ideally the user would receive an error and would not be able to save until an unused case id is entered.