Persisting flash when a form is submitted multiple times - ruby-on-rails

Question

Background: I have essentially a multiple choice quiz. When a user submits a form, update scores their answer, chooses a new question, puts a message in the flash, and redirects back to the show action. show displays the message and the new question.
The immediate problem is that if they submit the form twice, update gets called twice. The first time scores their answer for the question they were given. The second time scores their answer for the question that was just now selected, which they haven't seen yet. So then they get redirected with a message saying something like "wrong! The answer is Spain, not Italy" When they had been choosing between Hydrogen and Helium.
So my first solution to this is to include a question_id field in the submitted form. If the submitted id isn't the id of the question that was asked, just ignore this request. This works for keeping track, but the message that was stored in the flash is lost.
So I've tried adding flash.keep if the ids don't match. My thinking is that one update will set the flash, and then the next request received will be the second update. But the flash is still getting lost. It seems that the second update doesn't start with a flash at all, which confuses me.
So my questions are: is there a better way to handle this problem? And if not, why is the flash disappearing?
update looks something like this (some lines omitted):
def update
game = current_user.credence_game
question = game.current_question
if params[:question_id].to_i == question.id
given_answer = params[:answer_index].to_i
credence = params[:credence].to_i
correct, score = question.score_answer(given_answer, credence)
game.score += score
game.new_question
game.save
flash[:correct] = correct
flash[:score] = score
flash[:message] = question.answer_message(given_answer)
else
flash.keep
end
redirect_to action: 'show'
end
If I add a p flash line just before redirect_to, the server spits out this:
#<ActionDispatch::Flash::FlashHash:0x007f2a8431b4a0 #used=#<Set: {}>, #closed=false, #flashes={:correct=>false, :score=>-32, :message=>"Incorrect. The right answer is White River (Arkansas) (1102km) versus Yellowstone River (1080km)."}, #now=nil>
Started PUT "/credence" for 127.0.0.1 at 2014-08-17 15:20:16 +0100
Processing by CredenceController#update as HTML
Parameters: {"utf8"=>"✓", "authenticity_token"=>"EiqXDoFy77nmdKOE4YkHOeEFg4DvB5zdgFi98ynNCx0=", "question_id"=>"52", "credence"=>"60", "answer_index"=>"1", "commit"=>"60%"}
<Model stuff>
Redirected to http://localhost:3000/credence
Completed 302 Found in 667ms
#<ActionDispatch::Flash::FlashHash:0x000000059864f8 #used=#<Set: {}>, #closed=false, #flashes={}, #now=nil>
Started PUT "/credence" for 127.0.0.1 at 2014-08-17 15:20:18 +0100
Processing by CredenceController#update as HTML
Parameters: {"utf8"=>"✓", "authenticity_token"=>"EiqXDoFy77nmdKOE4YkHOeEFg4DvB5zdgFi98ynNCx0=", "question_id"=>"52", "credence"=>"70", "answer_index"=>"1", "commit"=>"70%"}
<Model stuff, just loading the game and the question>
Redirected to http://localhost:3000/credence
Completed 302 Found in 421ms
Started GET "/credence" for 127.0.0.1 at 2014-08-17 15:20:19 +0100
Processing by CredenceController#show as HTML
<Model stuff>
Rendered layouts/_messages.html.erb (400.4ms)
Completed 200 OK in 963ms (Views: 776.4ms | ActiveRecord: 27.7ms)
<GET requests for CSS, images, etc.>
So it does seem like the first request after the flash is set, is the one where I'm calling flash.keep, but it still doesn't have the flash.
Edit: I'm realising the problem is likely that I'm using CookieStore. The first request doesn't get a chance to set cookies before the second request is sent. Using the session instead of flash would have the same issue.
A simple solution here, that I don't much like, would be to simply disable the forms with javascript onsubmit. A more complicated one would be: when the question ids don't match, look up the answer that the user gave for the submitted question id, and set the flash appropriately without changing the score.
Any alternatives?