Issue with Deploying Rails App to Heroku

I am trying to deploy my Rails App to Heroku, but when I attempt to do so, this error message comes up in the terminal:

Your Gemfile lists the gem pg (~> 0.18.3) more than once.
remote:        You should probably keep only one of them.
remote:        While it's not a problem now, it could cause errors if you change the version of just one of them later.
remote: -----> Preparing app for Rails asset pipeline
remote:        Running: rake assets:precompile
remote:        I, [2015-11-10T21:51:48.354081 #1054]  INFO -- : Writing /tmp/build_2320e2913b6011e5babf7375ed5c6aab/public/assets/application-28d1fdfb204960e837089cdc7f8c27ebd6d792d449e7fffa162279ea245da036.js
remote:        rake aborted!
remote:        Sass::SyntaxError: Invalid CSS after "...m Stylesheet */": expected identifier, was "/**"
remote:        (sass):8938