“Double down” comes from Blackjack: you double your bet after seeing your first...

https://telegra.ph/Why-Do-Native-Speakers-Not-Notice-Idiom-Origins-07-29

“Double down” comes from Blackjack: you double your bet after seeing your first two cards, betting on getting a strong hand. Learners often think it means just trying harder, but it’s about risking more because you believe the odds are in your favour