Problem

You are getting the error:

bin/rails:6: warning: already initialized constant APP_PATH

when you are trying to start the local webserver with rails s.

Solution

That happens when are using the bootstrap rails gem as in:

gem 'anjlab-bootstrap-rails', require: 'bootstrap_rails'

It is important that the bootstap rails uses in the require is used with a dash and NOT an underscore.
So when you write the the gem as follows:

gem 'anjlab-bootstrap-rails', require: 'bootstrap-rails'

and restart your server you should no longer have the error message.