As I wrote, when you enter the system you must use a PIN to access voting. If you come online and find that you're blocked because someone has already used your PIN a backup security question (or more) could verify your identity, void the previous vote, and allow you to cast a valid vote. There WILL be problems, but can any of them be worse than the pay-to-play, lobbyist legal-bribery system we've got now? 

Do you really believe we'd be at war in the mid-east for the last 15 years at a cost to our children of $7 trillion if we'd had a popular vote in advance? Leaving such decisions in the hand of easily corrupted politicians is suicide, and makes us complicit by default with their criminal actions. If you've got a better way to make our government more democratic and responsive, let me hear it. Our two women warmonger senators are not the answer. They're part of the problem. If you've got another, less expensive solution, let me know.