Suppose there are eight people competing.
The single knockout is to divide these eight people into four groups, each group plays a round of competition, and the loser is kicked out. One person in each group will lose, and the remaining two will enter the finals after repeated rounds.
The double knockout is that these eight people are divided into four groups, continue to compete, be eliminated, and then be eliminated. In the end, there were only two people left, but the finals had not yet started. After the two men compete, the winner can enter the final directly (note "directly"). Everyone has to compete except the winner. After the game, the second winner is decided, and this second winner is eligible to play the final with the winner.
Last year's Super Girl had a so-called resurrection match, but the principle was similar to the double elimination system.