title: iPhone - testing if a notification exists
body: <p>At some point in a code one may add something like</p>
  
  <pre><code>[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(doSomething) name:@"Hello" object:nil];   
  </code></pre>
  
  <p>How do I test if this notification is already active on the queue or has been removed, to prevent adding a duplicate?</p>
  
  <p>thanks.</p>