Validation on drivers before assigning them on shipments

The case is when assigning a driver with power unit and equipment on shipment "X" while the shipment is running normal (Status Enrout_Enrout), while creating new shipment "Y" I can assign that same driver of shipment "X" on shipment "Y" ,So an unassign driver action occurs on shipment "X" and the driver with its power unit and equipment are assigned on shipment "Y" so I have no any validation at this time as any user can choose any driver any time.