Bagaimana cara menggunakan ON DUPLICATE KEY UPDATE di MySQL tanpa indeks UNIK atau KUNCI UTAMA?

INSERT INTO `table` (value1, value2) 
SELECT 'stuff for value1', 'stuff for value2' FROM `table` 
WHERE NOT EXISTS (SELECT * FROM `table` 
                  WHERE value1='stuff for value1' AND value2='stuff for value2') 
LIMIT 1

Sumber:Cara 'menyisipkan jika tidak ada ' di MySQL?