Iterator across all the non-duplicate keys and their values.
        
        Only yields the first key of duplicates.

def iteritems(self):
        """Iterator across all the non-duplicate keys and their values.
        
        Only yields the first key of duplicates.
        
        """
        keys_yielded = set()
        for k, v in self._pairs:
            if k not in keys_yielded:
                keys_yielded.add(k)
                yield k, v