Duplicate items when using update_attributes and decent_exposure

attached_file.rb

class AttachedFile < ActiveRecord::Base
  belongs_to :parent, :polymorphic => true                                                     
end

event.rb

class Event < ActiveRecord::Base
  has_many :attached_files, :as => :parent
 
  accepts_nested_attributes_for  :attached_files, :allow_destroy => true, :reject_if => proc {     |attrs| attrs.all? { |k, v| v.blank? } }                                                       
 
end

events_controller.rb

class Admin::EventsController < ApplicationController
  
  respond_to :html

  expose(:events) { Event.all }
  expose(:event)

  def create
    event.save
    respond_with(:admin, event)
  end

  def update
    event.update_attributes(params[:event])
    respond_with(:admin, event)
  end
end

new.html.erb

<% form_for [:admin, event] do |f| %>
  <%= f.text_field :name %>

  <%- f.fields_for :attached_files, event.attached_files.build do |a| %>
    <%=  a.text_field :name %>
  <% end %>

  <%= f.submit %>
<% end %>