Voting mechanisms are widely accepted and used methods for decentralized decision-making. Ensuring the acceptance of the voting mechanism's outcome is a crucial characteristic of robust voting systems. Consider this scenario: A group of individuals wants to choose an option from a set of alternatives without requiring an identification or proof-of-personhood system. Moreover, they want to implement utilitarianism as their selection criteria. In such a case, players could submit votes multiple times using dummy accounts, commonly known as a Sybil attack (SA), which presents a challenge for decentralized organizations. Is there a voting mechanism that always prevents players from benefiting by casting votes multiple times (SA-proof) while also selecting the alternative that maximizes the added valuations of all players (efficient)? One-person-one-vote is neither SA-proof nor efficient. Coin voting is SA-proof but not efficient. Quadratic voting is efficient but not SA-proof. This study uses Bayesian mechanism design to propose a solution. The mechanism's structure is as follows: Players make wealth deposits to indicate the strength of their preference for each alternative. Each player then receives an amount based on their deposit and the voting outcome. The proposed mechanism relies on two main concepts: 1) Transfers are influenced by the outcome in a way that each player's optimal action depends only on individual preferences and the number of alternatives; 2) A player who votes through multiple accounts slightly reduces the expected utility of all players more than the individual benefit gained. This study demonstrates that if players are risk-neutral and each player has private information about their preferences and beliefs, then the mechanism is SA-proof and efficient. This research provides new insights into the design of more robust decentralized decision-making mechanisms.

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