"Double down" comes straight from blackjack: you double your original bet with...

https://mike-wiki.win/index.php/Learning_Idioms_by_Observation_vs._by_List:_Why_One_Works_and_the_Other_Doesn%E2%80%99t

"Double down" comes straight from blackjack: you double your original bet with one more card, hoping to beat the dealer. In language, it means to commit more strongly to your choice or opinion