Bitcoin Forum

In a blockchain-based polling/voting system, what is the best way to ensure that nobody votes more than once, and what is the best way to keep votes secret (if that's a requirement)? The info available in the blockchain should be 1) who voted and 2) how many votes for each option.

How exactly do you mean this? For an altcoin? On top of the existing BTC? Forking BTC?

To accomplish this, you're going to have to figure out how to define a person.

You could theoretically issue each voter a single unit that can be used for voting, but that wouldn't prevent people from selling their votes to others.

You may find more answers here (https://cryptocointalk.com/topic/11079-proposed-block-chain-vote-to-switch-bitcoin-scrypt-from-proof-of-work-to-pure-proof-of-stake/).

Blockchains are generally the wrong tools for evoting— they don't solve any of the hard problems presented by that space.  They don't provide for censorship resistance (blockchains are not jamming free networks), they are generally privacy disasters (so they certantly don't help ballot confidentiality). They are not a sibyl free issuance mechanism, so they don't prevent parties from getting an unfair share of the votes.

I think electronic voting systems are exciting and interesting, but if you're using technology from Bitcoin to implement these systems you are likely using the wrong tools and will build something which lets people down.