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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _validate_singletons(self, boxes):
 """Several boxes can only occur once."""

        count = self._collect_box_count(boxes)
        # Which boxes occur more than once?
        multiples = [box_id for box_id, bcount in count.items() if bcount > 1]
        if 'dtbl' in multiples:
            raise IOError('There can only be one dtbl box in a file.')