“投资增加一倍”你可以直接说double the investment 即“加倍投资；将原投资变为两倍”,或者 raise/ increase the investment by 100 percent.by 后面跟的是一个比例或者具体的投资金额,但是不用 by one time!
“他们计划将投资增加一倍”
翻译1：They plan to double their investment.
翻译2：They plan to raise/increase their investment by 100 percent.