Nah, he'll tie himself on the hammer vote, so no auto-death, then each participant in the duel gets to select RPS, he'd just pick scissors for the hammer and rock for the lynchee, so he'd lose and owe himself a boon and we'd go into night phase. Day ends if the lynchee wins or loses their duel, only continues if the hammer refuses the challenge. Obviously we can't have people dueling themselves to indefinitely escape being lynched though, barring well coordinated agreements for two players to slap a vote on at nearly the same time... so the most obvious solutions is just to ban self-voting, or at least self-hammering during the game, I just want to think it over and make sure that doesn't open up some new can of worms before finalizing it.