Question: who has won back to back heisman trophies
[{'answer': 'Archie Griffin', 'provenance': [{'wikipedia_id': '16096383', 'title': 'List of Heisman Trophy winners', 'start_paragraph_id': 2, 'start_character': 949, 'end_paragraph_id': 2, 'end_character': 963, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}]