double start thread

例句：

1. 1.We need to double the number of workers for the project.
   我们需要为这个项目增加一倍的工人。
2. 2. The price has doubled since last year.
   自去年以来，价格已经翻了一番。
3. 3. He doubled his savings and bought a new car.
   他加倍了他的储蓄，买了一辆新车。