title: Firebase service named 'auth' already registered
body: <p>Get  this Error in console while using firebase and angularfire2 in my app</p>
  
  <pre><code> uncaught exception: auth: Firebase: Firebase service named 'auth' already registered (app/duplicate-service).
  </code></pre>
  
  <p>According to this answer
  <a href="https://github.com/firebase/firebaseui-web-react/issues/15" rel="nofollow noreferrer">https://github.com/firebase/firebaseui-web-react/issues/15</a></p>
  
  <p>I have check the code again but I am calling <code>firebase.initializeApp(...)</code>only one time in my <code>app.module.ts</code>. </p>
  
  <p>Now i am stuck and don't know where to start? Thank you in advance for any small help.</p>