Getting PDOException error

PDOException: SQLSTATE[23000]: Integrity constraint violation: 1062 Duplicate entry '66891-670-688' for key 'PRIMARY': INSERT INTO {quiz_node_results_answers} (question_nid, question_vid, result_id, is_correct, points_awarded, answer_timestamp, is_skipped, is_doubtful, number, tid) VALUES (:db_insert_placeholder_0, :db_insert_placeholder_1, :db_insert_placeholder_2, :db_insert_placeholder_3, :db_insert_placeholder_4, :db_insert_placeholder_5, :db_insert_placeholder_6, :db_insert_placeholder_7, :db_insert_placeholder_8, :db_insert_placeholder_9); Array ( [:db_insert_placeholder_0] => 670 [:db_insert_placeholder_1] => 688 [:db_insert_placeholder_2] => 66891 [:db_insert_placeholder_3] => 1 [:db_insert_placeholder_4] => 1 [:db_insert_placeholder_5] => 1378831829 [:db_insert_placeholder_6] => 0 [:db_insert_placeholder_7] => 0 [:db_insert_placeholder_8] => 36 [:db_insert_placeholder_9] => 0 ) in quiz_store_question_result() (line 2374 of /modules/quiz/quiz.module).

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