step up vs redouble

step up and redouble both are verbs.

Word	Noun	Adjective	Verb	Adverb
step up	No	No	Yes	No
redouble	No	No	Yes	No

As verbs, redouble is a hyponym of step up; that is, redouble is a word with a more specific, narrower meaning than step up:

step up (verb)	redouble (verb)
increase in extent or intensity	double in magnitude, extent, or intensity
speed up	make twice as great or intense
make oneself visible; take action	double again