How I remedied duplicate form submissions: https://www.tumblr.com/blog/ryankelley

application_controller.rb

class ApplicationController < ActionController::Base …

    private

    def client_will_not_cache_response
        response.headers["Cache-Control”] = “no-cache, no-store” 
        response.headers[“Pragma”] = “no-cache”
        response.headers[“Expires”] = “Fri, 01 Jan 1990 00:00:00 GMT” 
    end
end

puppies_controller.rb

class PuppiesController < ApplicationController

  before_filter :redirect_if_existing, only: [:new]

  after_filter :browser_will_not_cache_response, only: [:new]

  def create
    @puppy =
      Puppy.
        by_user_from_this_year(current_user.id).
        first_or_initialize(params[:puppie])

    ## prevent malicious user from setting the user_id
    @puppy.user_id = current_user.id

    if @puppy.errors.empty? && @puppy.save
      redirect_to show_puppy_path(@puppy)
    else
      ## Tell user they need to fix their mistakes
      render :edit
    end
  end

  private

  def redirect_if_existing

    ## If user already has a record in the puppies table
    ## created this year
    ## then redirect them to edit that Puppy.

    if Puppy.by_user_from_this_year(current_user.id).any?
      flash[:notice] =
        "You may only create one puppy per year. You may edit your existing puppy."

      redirect_to edit_puppy_path(@puppy)
    end
  end
end

I created this as an anonymous user at first, annoyingly enough:
original