The Complexity of Cloning Candidates in Multiwinner Elections

We initiate the study of cloning in multiwinner elections, focusing on single-transferable vote (STV), single-nontransferable vote (SNTV), bloc, k-Borda, t-approval-CC, and Borda-CC. Transferring the model of cloning due to Elkind et al. [15] from single-winner to multiwinner elections, we consider decision problems describing possible and necessary cloning in the zero-cost, the unit-cost, and the general-cost model and study their computational complexity. We show that, depending on the multiwinner voting rule and on the cost model chosen, some of these cloning problems are in P, some are NP-hard, and some of the latter (for which, in fact, already winner determination is NP-hard) are fixed-parameter tractable.