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

https://stephensnicejournal.almoheet-travel.com/what-idioms-do-examiners-dislike-a-dry-dive-into-english-exam-register

“Double down” comes from blackjack, where you double your bet after seeing your first two cards—bold move, with high risk and reward. Learners often misuse it to mean “do something twice,” but it’s about committing more when stakes get real.