In reply to mav:
> I guess you could have all voters given an ID (their NI number) and then you could log all voter ID's used at each polling station and drill into the data. But we are a good way from that.
I consult on elections in many developing countries, and multiple voting is usually a big issue, albeit in practice much more a perception of unfairness rather than seeing actual abuse widely enough to be potentially result-changing. To be fair though, the actual cases of multiple voting are often impossible to quantify.
There are plenty of IT systems around that can easily prevent this, albeit at considerable cost. Each, however, also brings with it other issues such as reduced trust in the voting and counting system once more is being done electronically.
The 'easy' way to find out whether multiple voting actually happened would be to data enter every recorded voter who was issued a ballot. This would be hugely expensive, but more than that, it would create an electronic database of people's voting habits and locations, which itself could be very much open to abuse, so wouldn't be a course of action to be undertaken unless there really was evidence of very widespread multiple voting.
On a more preventative level, as long as all polling stations are data linked, it would be very possible to implement a system whereby someone with more than one registration coming to vote will trigger a notification to all other places they are registered, but even that is not without problems and potential disenfranchisement.
The only thing that can really be achieved relatively easily and safely is to limit the number of multiple registrations in the first place, and it's difficult to see a justification for more than two for any person. Timing of late registration may be a cause and really voter cards should not be sent out until the register is finalised. In my experience though, no system is perfect and there are usually good reasons why apparent anomalies are allowed to persist, which is not to say we should be complacent, but perhaps not too trigger happy in jumping to conclusions of widespread abuse of perceived loopholes.