On a recent flight to SIN I used 5 +a £10 preorder discount to buy £88 of goods for £28. (Family of 5). Returning on a SYD-BKK flight CSD came to see me to advise that they could only accept 1 as they had the same code - I think I could have got them to put it through as 5 separate transactions but thought it not worth the aggro. Same crew on the BKK-LHR flight several days later so I didnt bother.