Is there a way to tell Rails just to include it at most once? kind of like the PHP ways of require_once or include_once
If it's already included in the layout, you won't need it in the partial. If you are wanting to customize the js loaded for each page, so you can better control load times, I would look into the content_for method.
A great explanation can be found here http://guides.rubyonrails.org/layouts_and_rendering.html in sections 3.2 and 3.3
Hope this helps.