Simple majority vote is the only procedure which is anonymous, dual, and monotonic.
May's Theorem
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References
May, K. "A Set of Independent Necessary and Sufficient Conditions for Simple Majority Decision." Econometrica 20, 680-684, 1952.Referenced on Wolfram|Alpha
May's TheoremCite this as:
Weisstein, Eric W. "May's Theorem." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/MaysTheorem.html