Thanks to this dialog:
https://github.com/rails/jquery-ujs/issues/208
I was able to discover that jquery and jquery_ujs were getting included twice.
I guess the jquery-rails gem automatically puts them into application.js, and then I had them included in application.js as well.
Seems like for whatever reason application.js automatically bundles everything in app/assets/javascripts/ – even when I remove all the requires.
So, if your :remote => true forms are getting submitted twice, try checking application.js.
Hope this helps!
UPDATE: I believe this might have had something to do with me pre-rendering my assets without using a digest, so then when my development environment adds script tags in the html head dynamically from the requires in app/assets/application.js, it also adds one for what should be an empty dynamic application.js, except the static one from public/assets gets loaded. Confusing? Yep!