title: Firebase Android onAuthStateChanged called twice
body: <p>I've start working with new Firebase SDK.</p>
  
  <p>When I'm doing user login, I'm onAuthStateChanged method is being called twice with same state (etc. user sign in).</p>
  
  <p>I'm sure I'm adding the AuthStateListener only once to the FirebaseAuth reference.</p>
  
  <p>Any help?</p>