title: Auth state change listener being fired twice?
body: <p>When my page loads I need to know whether the user is logged in and perform different tasks based on that. The first thing I do is check if the they are logged in, but for some reason the <code>if</code> statement to check that is being validated twice. Here is my code:</p>
  
  <pre><code>override func viewDidAppear(_ animated: Bool) {
      FIRAuth.auth()?.addStateDidChangeListener { auth, user in
          if let user = user {
             print("User is logged in")
          }
  </code></pre>
  
  <p>"User is logged in" gets printed out twice to the console when the page loads instead of just once. Am I doing something incorrectly?</p>