Hair should not be dyed twice in one week. This will expose the hair to the harsh chemicals two times in a short time frame. The general rule is to wait at least 4 weeks before coloring the hair again.
The condition of the hair after the first color change is important. Hair that is damaged after being colored should not be colored too soon. The focus should be to restore the health of the hair. If the dye is of a darker color, it will be less damaging to the hair. It should be fine to dye the hair before the recommended 4 weeks.