To summarize:
The issue I had was due to the fact that I once precompiled my assets and then decided to compile on the fly:
- It's not handy for debugging in dev mode
- Heroku manages precompilation automatically when pushing the code
The precompilation created minified version of the assets in my public folder.
When I run my website, all the javascript files were then loaded twice: Once from the public folder and once from the normal flow.
I just deleted what I had in my public folder and it solved my issue.