title: Swift - notification observer is called several times
body: <p>I have viewController and inside in viewDidLoad I have</p>
  
  <pre><code>NSNotificationCenter.defaultCenter().addObserver(self, selector: "showNextQuestions", name: "showNextQuestionsID", object: nil)
  </code></pre>
  
  <p>In another controller I have</p>
  
  <pre><code>NSNotificationCenter.defaultCenter().postNotificationName("showNextQuestionsID", object: nil)
  </code></pre>
  
  <p>If I go home from app and launch it again function showNextQuestionID fires two times.</p>
  
  <p>I tried to use </p>
  
  <pre><code>func applicationDidEnterBackground(application: UIApplication) {
      NSNotificationCenter.defaultCenter().removeObserver(self, name: "showNextQuestionsID", object: nil)
  }
  </code></pre>
  
  <p>But this doesn't help,</p>
  
  <p>and in viewController</p>
  
  <pre><code>deinit {
      NSNotificationCenter.defaultCenter().removeObserver(self)
  }
  </code></pre>
  
  <p>How can I fix this ?</p>