Just assign the existing attachment to another record:

new_photo = Photo.new
new_photo.image = old_photo.image

Paperclip will duplicate the file when saving.

To use this in forms, pimp your attachment container like this:

class Photo < ActiveRecord::Base
  has_attached_file :image
  
  attr_accessor :copy_of

  def image_url
    if copy_of
      copy_of.url
    else
      image.url
    end
  end
end

And in the controller do:

new_photo = Photo.new(:copy_of => old_photo)

This is more tricky than it should be because url_for, asset_path, etc. all rely on different mechanisms.

Anyway, you can use the attached trait like this:

class ExampleController < ApplicationController
  does 'host_enforcement', :for => 'some_action'
end

Short explanation:

• asset_host is used for links to stylesheets and javascripts
• asset_host belongs to ActionController::Base -- changes are persistent and will not be reset after a request
• rewrite_options is used by the ..._path methods in the views
  ^...