“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, betting you’ll win big. Learners often say “double up,” which sounds right but means something else—usually gaining twice as much