You can, but it's not recommended. The new grout will not adhere well to the old grout, and it is likely to peel off over time. It's best to remove the old grout before installing new grout.

Yes, you can put new grout over old grout. But first, you should clean the old grout to remove all the dirt and debris. Then, let the surface dry completely before applying the new grout. Make sure to follow the manufacturer's instructions for mixing and applying the new grout.