title: Remove all strings with duplicates in an NSArray
body: <p>I am trying to figure out how to implement this in Objective-C. </p>
  
  <p>I want to remove the strings in an <code>NSArray</code> that have appear more than once in the array. </p>
  
  <p>At the end I want to have an array that only has the unique lines in an array (meaning that not just the duplicates are deleted but also the original string that matches the duplicates.) </p>
  
  <p>For example if you had the following array:</p>
  
  <pre><code>NSArray *array = [NSArray arrayWithObjects:@"bob", @"frank", @"sarah", @"sarah", @"fred", @"corey", @"corey", nil];
  </code></pre>
  
  <p>I would want the new array to look like this:</p>
  
  <pre><code>@"bob", @"frank", @"fred"
  </code></pre>