Extraneous Package when Installed Locally

I was trying to install phantomjs in order to make test Twitter Bootstrap. After I had installed it locally i.e. npm install phantomjs, it reported that the package was extraneous.

├─┬ phantomjs@1.9.0-3 extraneous
│ ├── adm-zip@0.2.1
...
npm ERR! extraneous: phantomjs@1.9.0-3 /Users/admin/bootstrap/node_modules/phantomjs
npm ERR! not ok code 0

However, when phantomjs was installed globally i.e. npm install phantomjs -g, it worked fine i.e. no extraneous error reported.

Questions:

1. Is it because phantomjs not specified in the package.json file?
2. General question: Can we have any package e.g. phantomjs installed globally and also locally?

1. Yes. (Re-installing with npm install wont install phantom.js again.) (Btw.: npm install xxx --save will automatically add xxx to the package.json)
2. Yes. Local package versions are preferred over global ones. (Although you need some path handling for executables.)

1. phantomjs should be included in your local .json package(manually editing it or using --save command see TheHippo's answer). If it is installed somewhere and you don't need it use the prune command

npm prune

will remove all non-required packages.

2. To install an item both locally and globally use:

   sudo npm install -g phantomjs