I admit that on maybe a couple occasions in the past I've used the return portion of a FOR more than once... In the good old bad old MML days (I'm talking early post privatisation, not during MML's final days) when neither St.Pancras nor Nottingham were barriered, there were several occasions when my Used ticket wasn't marked. Usually there'd be a manual check at the head of the platform at St.Pancras, or staff onboard the train would mark the ticket, but sometimes neither would happen and I'd leave the station at Nottingham with an unmarked portion of a FOR ticket still in my possession. At the time, I was travelling at least weekly and often more than that, so the opportunity to use the return portion of that unmarked FOR again would have been (and once or twice was) all too easy - arguably an undetectable opportunity to save money that could be taken with impunity and without fear of comeback.
Whilst I'm very aware doing so was wrong, I also have a view that any company has a duty of care in respect of protecting its income, and I don't have that much sympathy for the company in the circumstances I've outlined above. Again let me emphasise I never attempted to evade or avoid ticket checks, but rather on a couple of occasions I took advantage of an oversight or omission on the part of then train company's staff that resulted in me being able to reuse a ticket. If the company's staff - who always checked first tickets on board to ensure only first ticket holders were seated in first - were too stupid or lazy to draw a line through a ticket they were checking, then I used to think that maybe the company got all it deserved in terms of lost revenue. MML should in those early days have been using every opportunity and effort to cancel used tickets, especially some of the most expensive it issued.
The situation I described is almost impossible now - both stations are barriered and EMT staff are very diligent when it comes to cancelling used tickets during onboard checks. I can't remember ever even having an opportunity to consider doing the same since EMT came in. And besides, if such an opportunity did arise now, rather than attempt to reuse the ticket I think I'd send a "complaint" about the on-train staff to the company's management, explaining that those staff were potentially losing the company money by failing to cancel expensive tickets that could otherwise possibly be reused.
Last week I was in an upmarket restaurant in a group and they charged for seven drinks instead of the eight we'd consumed. Did I tell them about their mistake - no, sorry, I didn't! There was no attemp on our part to deliberately avoid paying for all we consumed, but the staff were too lazy to keep track of everything we'd ordered. Part of me says this reusing uncancelled tickets thing is similar. I guess that what I'm saying is that if the guy on TripAdvisor was reusing an used, uncancelled but otherwise valid (i.e.indate, on route, correct time restrictions, etc.) ticket, then yes I agree it's wrong - but more fool the TOC for not protecting its own revenue properly.
Andy