“Double down” comes from blackjack: you double your bet after seeing your first...

https://oliviasgreatop-eds.fotosdefrases.com/idioms-that-sound-negative-but-are-actually-neutral

“Double down” comes from blackjack: you double your bet after seeing your first two cards, risking more for a bigger win. Learners often say “double up” instead, which means something else entirely. Use “double down” to show commitment, not just increase.