For this race, Londoners used an RCV system, in which they were permitted to rank their first and second choices. But a terrible, confusing ballot design led a significant number of voters to cast a first choice for two different candidates — resulting in their ballots being tossed out. This is a problem of ballot design, not inherent to RCV. But I would not be surprised to see this debacle being used in arguments against RCV. Here’s the ballot: