It's 1970s tech. Even if they had given them unique codes, they had no way of catching duplicates. The magnetic stripe has a very limited amount of storage space and, as far as I know, it's all used for other necessary information.

It also explains why the rail operators are moving to QR codes on paper tickets, which are in every respect worse. (They are absurdly large tickets and take much longer to scan at the barrier, creating queues.)