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!