Can a newborn be named "Muḥammad" with a ḍamma, or is this designation reserved exclusively for the Prophet, peace and blessings be upon him?

There is no doubt about the virtue of naming a newborn Muhammad, as it is the name of the master of the children of Adam. It has been reported that the Prophet, peace and blessings be upon him, said: "Indeed, you will be called on the Day of Resurrection by your names and your fathers' names, so make your names good." If your sister means to pronounce the first mim of the name with a ḍamma (Muḥammad), then this is correct. As for pronouncing the second mim with a ḍamma, it is considered a grammatical error, and it is better to avoid it, so that the newborn may be honored with the name of the Seal of the Prophets, Muhammad, peace and blessings be upon him.