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

https://dlf-ne.org/is-up-the-ante-a-gambling-term-unpacking-a-classic-business-idiom/

“Double down” comes from blackjack, where you double your bet after seeing your first two cards, hoping to beat the dealer. Learners often say “double up” instead—wrong move, linguistically speaking