A Double Voter is a role that has the ability to cast two votes. Usually one vote is done in thread, and the other is privately told to the mod and is placed anonymously. The same player does not necessarily need to be double-voted, either. Double voters are not often announced in thread, so it's up to the players to pay attention to vote counts and see if they seem off. If they do, players can request mod confirmation that the vote count is correct.
Double voting can be either part of the player's role, or can be paired with another role, a vote stealer. In either case, both roles are rarely mafia roles, as giving the mafia an extra vote can make it easier to eliminate the town.