The answer is yes, you can always double down in blackjack. You are allowed to make two additional bets, and if you are able to win both bets, you will receive the original wager plus the additional wagers.
However, there are a few caveats that should be noted. First, if you are able to win one of the bets but lose the other, you will only receive the original wager.
Second, if you are able to win both bets, but only because the dealer has dealt a blackjack that is not a ten, then you will only receive the original wager and not the additional wagers.
In general, it is always best to play conservatively when playing blackjack – even if that means foregoing the opportunity to double down. By playing cautiously and keeping track of the odds, you can increase your chances of winning without risking too much money.
