H,

  Please make appropriate changes.

You can refer 

modelObj = self.env ['hr.employee']

        for employee in self:

            employee_id = modelObj.search ([('name', '=', employee.name), ('passport_id', '=', employee.passport_id)])

            if employee_id:

                employee.write ({

                    'sequence number': employee_id [0] .sequence_number

                })

            else:

                 sequence = self.env ['ir.sequence']. next_by_code ('new_code1')

                employee.write ({

                    'sequence number': sequence

                })