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

https://idiominsider.com/talking-odds-at-work-the-risk-idioms-every-english-learner-gets-slightly-wrong/

“Double down” hails from blackjack, where you double your bet after seeing your first two cards—bold, risky, but potentially rewarding. Learners often misuse it as ‘double up’ for any increase