I noticed the -1 note next to my season card when I just logged in now to check availability. Wondered what it meant, but if they have now stopped people buying more than one per SC that would explain it. My other card - used only to buy one ticket - now says '0' therefore meaning it can't be used to buy additional tickets. I assume the -1 is just some quirk of the IT system behind the ticketing (like a form of stock adjustment, not a recall of the ticket). It may also be used to create a database for identifying potential touting behaviour as it provides a sample from which to trace those which end up on the black market and take appropriate action.
Doubt they will attempt to 'cancel' the purchases already made for the additional tickets (although I also remember them doing it a few years ago). This is mainly because they could generate immense anger among those who put the most into the gate receipts each season. Similarly some people would still chance turning up at the box office with their original e-mail on the day saying they hadn't received their paper ticket, and playing 'dumb' when told it has been cancelled ("I didn't see any e-mail canceling it"). Imagine the angry scenes. Policing big European nights is hard enough without potentially several thousand ticket-less people converging on the box office and surrounding areas. It is also conceivable that if two owners of one seat both got into the ground it'd be a stewarding nightmare. It could also cause chaos at the Stoke game at the weekend with lots of angry people looking to air their grievances. So although they may want to go back in time and do it differently I'd be surprised (and very annoyed) if they did.
I personally bought 3/4 tickets my two ST were eligible for. We have two SC which we share between three of us in my family, with different combinations going to each home game. Incidentally we also pay for one Cityzens membership each year to try and get priority if we all can and want to attend a game, and that person (although it is all three of us if you understand me correctly) will have the extra ticket. I think it is situations like which underpinned the decision to offer two tickets per card, in tandem to the lack of sales for earlier rounds. I agree it should have been one, but I do genuinely think the majority who have bought more than one have done so in good faith, not to piss others off. Anyone who has bought more than one out of greed (i.e. because they want to sell it, or just because they could, not because they had a good use in mind) should hang their heads, as they are the ones spoiling it for everyone else.