title: Keys in NSDictionary can be duplicated?
body: <p>From what I have read, keys in dictionaries are unique.</p>
  
  <p>Consider this code:</p>
  
  <pre><code>NSMutableDictionary *mydic = [NSMutableDictionary dictionary];
  
  [mydic setObject:@"value1" forKey:@"key1"]; 
  [mydic setObject:@"value1" forKey:@"key1"];
  [mydic setObject:@"value1" forKey:@"key1"];
  </code></pre>
  
  <p>Why can I run this without any error? What should I do to avoid duplicate keys?</p>