Usernames are not unique. Anyone can name themselves anything they want, as well as rename themselves whenever they want. As a demonstration, go to Users and search for John. You'll have hundreds of hits.
The system instead uses user IDs as the authoritative reference. For example, you are user 772394, and I am user 208574. Thomas Ward (the mod) has the user ID 10616. The user ID is a part of the URL of a user account:
In theory, it becomes possible then for a user to have the same name as a moderator. To prevent cases of mistaken identity, moderators are distinguished by the ♦ by their name as well as the aura of power emanating from their user cards.
The moderator Thomas Ward will have their diamond, and will look something like this on the site:
Otherwise, usernames are almost a free-for-all. Diamonds, as well as some other characters, may not be used in usernames (for obvious reasons).