It's NOT a vote - it's a nomination. And repeat nominations make no difference. Here is a quote from the leaflet ;-
"The Bank is seeking nominations so the Banknote Character Advisory Committee can consider a wide range of potential characters. The decision will be based on the strength of the contribution of the nominees, not on the number of times they were nominated, so please do not send multiple nominations for the same character. It is possible to nominate more than one individual."
The nomination I submitted several weeks ago is on the list.