What is the linguistic structure of the word "Allahumma"? Is it one of God's Most Beautiful Names, and if not, why is it used at the beginning of supplications such as "Allahumma, forgive me"? Does this imply a supplication to another god named "Allahumma"?

The origin of "Allahumma" is "Allah", and the doubled meem at its end is a substitute for the vocative particle "Ya", according to the majority of scholars. Al-Baghawi holds that its meaning is "Ya Allah", and the meem was added after the vocative particle was removed. The Kufans, however, believe its origin is "Ya Allah ummanā bi-khayr" (O Allah, lead us to good), then the hamza was omitted, and they think it is sometimes permissible for the vocative particle to precede it. As for the Basrans, they believe the meem is a substitute for the vocative particle and it is not permissible for them to be combined. Some grammarians go further, stating that whoever says "Allahumma" has invoked Allah by all His names, and that it combines supplication.