How I remedied duplicate form submissions via preventing cache, and preventing duplicate data save in database in controller layer.

application_controller.rb

class ApplicationController < ActionController::Base
  prepend_before_filter :authenticate_user!
  private

  ## Tell client not to cache the response
  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 :load_puppy, 
    only: [:new, :create, :show, :update, :edit, :delete]
  
  after_filter :browser_will_not_cache_response, only: [:new]
  
  def new
    ## If user already has a record in the puppies table 
    ## created this year 
    ## then redirect them to edit that Puppy.
    if [email protected]_record?
      flash[:notice] = "You may only create one puppy per year. You may edit your existing puppy."
      redirect_to edit_puppy_path(@puppy) and return
    end
  end

  def create
    ## 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 load_puppy
    @puppy = 
      Puppy.
        includes(:user).
        by_user_from_this_year(current_user.id).
        first_or_initialize(params[:puppy])
  end
end 

Rails: How to Prevent Submitting Duplicate Forms

How to Prevent Submitting Duplicate Forms PT. II