Double vs. redouble

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Grammarist

To double something is to make it twice as great. To redouble is (1) to double something again, or (2) to make something much greater (as opposed to just twice as great). In modern usage, the second sense of redouble is more commonly used than the first.

Double and redouble have related Middle-French origins, but they have had these separate meanings for as long as they’ve been in English. And both are old; double entered the language around the the 14th century,1 and redouble came century or two later.2

Examples

In these examples, double means to make twice as great in size, number, or intensity:

China’s economy will double in seven years, and its military budget will double in approximately five years. [National Review]

The number of American soldiers who lost at least one limb doubled from 2009 to 2010. [NPR]

October 31 will see the human population reach an ominous milestone after doubling in less than 50 years and threatening an environmental catastrophe. [Express.co.uk]

And in these examples, redouble means to make much greater:

 Instead, today’s survey should be an immediate wake-up call to redouble our efforts in promoting our wonderful city. [Scotsman]

Ever since Obama redoubled his push to hike taxes on the rich, conservatives have been ridiculing Obama’s invocations of fairness. [Washington Post]

He said the agency was considering redoubling efforts to inform people about how to travel safely in grizzly country. [New York Times]

References

1. Double in the OED (subscription required)
2. Redouble in the OED