I had a problem like this before. It was caused after I had precompiled the assets it was going after the applcation.css inside the public folder as well as in the apps directory. I’m not sure how to fix it so that it doesn’t keep happening while in dev mode but if you delete your /public/assets directory it should fix it.
Check and see if you have a public/assets folder, if you do and it’s full, it’s probably why you’re seeing double.