Unfortunately, that is a bug with the voting plugin that is used. When I was on the mod team, we spent a considerable amount of time trying to find the cause, including working with plugin developers over on https://meta.discourse.org, but couldn’t find and therefore resolve the cause.
The biggest thing people can do to prevent wasted votes is to search for and use existing topics.
Edit: Speaking of which… Jayne has just posted an update in News and Announcements on this issue. I’ll link it here for those of you who haven’t subscribed to topic notifications when new topics are created in that category: An Update on Voting!