iPhone - testing if a notification exists
<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>
