I cannot see how that can be. Can someone explain in what sense flagging and then voting would be double voting?
Flagging raises awareness; to my knowledge it does not influence the voting process on the question. In effect it is a polite systematised alternative to posting on Meta or asking for close votes in chat. No one would think that raising a Meta question about whether a question should be closed would give you two votes, so I don't see how flagging could be considered double voting either.