"Double down" comes from blackjack, where you double your bet after seeing your...

https://www.4shared.com/office/12SjSzBTge/pdf-11845-28864.html

"Double down" comes from blackjack, where you double your bet after seeing your first two cards, aiming for a big win. Learners often misuse it to mean 'do something twice'—not quite. It’s about commitment, not repetition