Although it is possible to apply fresh grout over existing grout, doing so is not advised. The new layers and the older layers do not cling to each other; this is only a stopgap measure. There is a good chance that the same issue may arise again.
In order to effectively address issues such as grout cracking and chipping, as well as any other concerns, the previous grout must first be removed. In order to make the surface suitable for the new grout to adhere to, it is necessary to meticulously wipe away any dust and debris that may be present. During the process of application, the grout has to be pushed firmly into the cracks to ensure that there are no spaces that might prevent the adhesion from reaching its full potential.