the O.P. in the original thread (remember that??)
did claim to have proof of purchase.
Proof of payment is not a ticket and does not authorise travel, or entitlement to a refund or compensation.
When I make rail journeys on business I get a receipt for the ticket - and the ones I get at Stalybridge actually describe the ticket that I bought.
Receipts, however, do not describe the person using the ticket or which train they are travelling on. Which is why they are not an authority to travel.
So (and this is entirely possible) suppose I get on the train on Monday (as usual), put my ticket in my top pocket (as usual) and my receipt in my wallet (as usual) and the guard comes round but for some reason I can't find my ticket so he sells me another one .....and then I do find my original uncancelled ticket and of course I also have my receipt ..... I can prove pretty much beyond all shadow of a doubt that I have purchased two tickets for one journey and can show all the documents there and then. Would the guard cancel the second ticket and give me my money back?
In such circumstances I would fully expect the guard to non-issue the ticket that has been sold, although experience tells me that usually they will advise going to the ticket office for the refund, where the refund will be given (albeit not usually immediately).
However, these are not the circumstances under discussion at the start of this thread. In those circumstances, where there is no evidence that the original ticket was held at the time of travel and a replacement was purchased at that time, any refund on subsequent production of the original ticket is a matter of goodwill. Such a refund would very much depend on the type of ticket originally purchased and whether it was refundable, but I have authorised refunds in such circumstances in the past.