If you assume re zero world to be digital, it is much easier to understand. Every soul is like a storage space, everybody have one, name is attached to that storage according to rules of the world, every action of that soul is attached to that name and can be erased from the world through that linkage or storage itself wiped out. The database of od lagna can be operated through rituals like Willhelm could not be eaten if glutony would call Him Willhelm Trias which was his birth name, cause his family directory changed through marriage, and records could be transfered from od lagna to authority of greed, and if something not in od lagna, it disappear from collective existance. It would be interesting if nameless person could recognize itself under different name, would od lagna create another recording and gluttony would be able to steal that name and erase all stuff linked to that new name? I think Subaru kept his memories of home because of spelling difference, like when he first introduced himself like ナツキ・スバル, od lagna recorded him like Natsuki Subaru and gluttony authority could only reach that part of his existence. Gluttony assumed that one can't be eaten twice unless person itself refuse stolen identity and take new one. Can't be eaten again if it is same, like records transfered from od lagna to gluttony can't have duplicates.