The word "re-double" means to increase or intensify something. Some antonyms for "re-double" are reduce, decrease, diminish, weaken, and lessen. These words indicate a decrease in intensity or strength, rather than an increase. For example, if someone is asked to re-double their efforts, they are being asked to work harder or more focused. An antonym for that would be to lessen the efforts, or work less hard. Similarly, if someone re-doubles the volume on a speaker, the opposite would be to decrease the volume. In summary, antonyms for re-double represent the opposite of increasing or intensifying something.