link_to :confirm displays popup twice

It sounds like the confirmation handler Javascript in rails.js is being attached twice.

Could you be accidentally including two copies of rails.js via duplication of a javascript_include_tag helper?

I was having this same issue, and spent the better part of an hour looking into it. I found a solution in my situation, and I figured I would share it in case it helps...

My problem was that I had

config.assets.debug = true 

in config/environments/development.rb

Changing that line to

config.assets.debug = false

fixed the confirmation duplication for me. I found the relevant information in this rails guide to asset pipeline. Hope it helps!