In Quiet there is no global naming. A community is joined through an invite link (a bundle of onion addresses) so multiple communities can have the same name. Usernames are unique only within communities, and we use certificates signed by the community creator as a single source of truth on names. However, in order to allow joining when the community creator is not online, we will release changes this week that let users join provisionally with any name, even duplicates. We will show an 'unregistered' badge until registered and a 'duplicate' badge if duplicate. We will also show an aggressive warning to everyone if the registrar is malicious and gives multiple public keys the same name.
It seemed important to get unique usernames within a community because users expect that from apps like Slack and Discord.