double与twice的用法和区别

double与twice的用法和区别主要是讲到倍数的时候,如：Khone Falls on the Mekong River in Southeast Asia sends much water over the edge every second — nearly double the flow of North America's Niagara Falls.nearly double在这里如何理解?是“将近一倍”还是“将近两倍”?