I'm hoping this is a "quick fix", but it's far from a priority. A small change might help a lot here.
I got some egg on my face yesterday when I thought there was a bug with old election pages showing vote buttons. A new user thought the same thing and had asked in chat. It turns out that's by design
Look at the candidates you voted for. The '1st', '2nd' and '3rd' choice buttons look different there; here are the buttons on the candidate I voted for as second choice:
Unless I get shown a candidate I did vote for, my screen looks like this
While voted candidates show like this (note 1st and 3rd are greyed out)
I would like to see all the buttons, save ones I used to vote for, shown as greyed out. It would make it visually obvious that one cannot cast a new vote.