Conditional path names in Paperclip

Question

I'm using this Paperclip configuration:

  has_attached_file :image, :storage => :s3,
                    :styles => { :medium => "600x600>", :small => "320x320>", :thumb => "100x100#" },
                    :s3_credentials => "#{RAILS_ROOT}/config/s3.yml",
                    :path => "/:style/:filename"

This gives me nice-looking paths, but it breaks when two files have both the same style and filename (i.e., one file clobbers the other)

I want it all – nice-looking paths when filenames are unique, and no clobbering when they're not. I.e., I want to determine the path like this:

  def path
    if filename_is_unique
      "/:style/:filename"
    else
      "/:style/:id_:filename"
    end
  end

How can I accomplish this within Paperclip?