Since Mohammad is spelt in Arabic with double M, the English sentence should not report a misspelling when it is spelt that way.
You used to follow the concept of writing double letter where there is stress. So based on this and the rule you use "محمّد" should be writen "Mohammed" and not Mohamed".
Would you put a comma in Arabic where there can be a comma in English? (This is my brother, Mohammed - "هذا أخي، محمد")
How can i say
Muhammad, this is my brother.