title: Why having the same NPM package declared in regular and dev dependency is considered bad?
body: <p>NPM triggers a warning message: <code>Dependency '...' exists in both dependencies and devDependencies</code> when my <code>package.json</code> explicitly declares the same module in both <code>dependencies</code> and <code>devDependencies</code>. It suggests that the NPM's developers think it is a wrong way of using NPM.</p>
  
  <p>This is surprising to me since I've always considered it a good practice Doing so means I can change (add / remove / update) my "app dependencies" without breaking my build scripts etc.</p>
  
  <p>Do you know the rational behind this view or do you have examples where it could introduce problems?</p>