If you plant cotton, then water it cuts the time in half. If you wait 24 hours then water the same crop again it doesn't cut it in half the second time. It only works once per crop as you say.
However if you plant raspberries, water, harvest and replant raspberries the same day it will cut the time in half on the first planting, but not on the second planting. It only works once per 24 hours.