A user reported this problem: http://feedback.textasticapp.com/topic/144743-cannot-vote-says-ive-already-voted/
When I open http://feedback.textasticapp.com on my iPad and tap on the vote box, I also get the message "You had already voted". This happens whether I'm signed in or not.
Also, it should be easier to add a vote on iPad/iPhone. Right now you have to hit the tiny arrows with your finger to add your vote.