Doubling Down in Blackjack.?

NEVER split ten value cards (this is what you are asking about, Doubling down is something different). You have a much higher chance of winning with a 20 than with two 10s. Unless the dealer is "stiff", meaning a 4, 5, or 6 showing, you only want to split As and 8s. Other splits are based on what the dealer has. Do a search for "Blackjack Basic Strategy". This will show you when splits are warranted.