Comment by warbiscuit

Ethereum's Casper uses the solution that while validator A could double-vote for multiple blocks, validator B could take those multiple vote messages, and submit them as cryptographic proof of A's dishonesty. By submitting such proof, the protocol would allow slashing A's stake, and giving some portion to B as a reward for catching them. Thus validators are strongly incentivized to only vote for one block at the end of the chain.