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

https://wiki-book.win/index.php/059_Gambling_Vocabulary_That_Became_Mainstream_English

“Double down” comes from blackjack, where you double your bet after seeing your first two cards, hoping for a strong finish. Language learners often confuse it with ‘double up,’ but it’s about committing more, not just repeating