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

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

“Double down” comes from blackjack, where you double your bet after the first two cards, hoping your next card hits big. It’s about commitment, not just repeating something twice. Learners often say “double up” instead—close, but different game entirely