The SmartGate passport tech could easily be adapted to confirm voter ID and prevent multiple voting attempts.
No, it couldn't. Research has shown over and over again that if you have time alone with the hardware you can always find a way to subvert it. Smartgate works fine in the airport because there are always about a thousand other people looking right at you while you're using it. The nature of voting is that it is secret, so you by definition have to be alone with physical access to the machine. And that's just the vulnerability to the person walking in to vote. There are numerous other avenues of attack that could be taken by various people in trusted positions.
I'm afraid for the time being at least, paper votes counted by hand while a lot of suspicious people watch remains the most secure method of doing it. Also, gratuitous XKCD.