Hi, hornbeam. Sorry to hear about your problem.
I'd suggest you do phone them again tomorrow morning: make sure you get the name of the person you speak to (did you get the name of your previous contact?) and, if you feel you're not getting anywhere, ask (politely but firmly) to speak to their manager.
I agree on the photo ID point: if the duplicate season tickets are registered to the same photo ID, any ticket inspection should pick up anyone else who might be mis-using your 'lost in the post' returned ticket?
However, I'm not
FGW▸ staff, so I don't know for sure: perhaps one of our very knowledgable staff members could confirm?
(
cough) Ollie?