Rails 3 Cocoon link_to_add_association Renders Partial Twice

I was having this same issue. My problem was I requiring the cocoon javascript twice.

Once in my application.js

// app/assets/javascripts/application.js
//= require cocoon

and once in my application layout

/ app/views/layouts/application.html.haml
= javascript_include_tag :cocoon

after removing the include tag from my layout it began working as expected