I just took a look at the users-page. I am surprised that we can have a couple of users with the same username. Is that by design/wanted or a bug? Or something else? See userpage for "math"
Note: I don't know whether "bug" is an appropriate tag...
To borrow from the accepted answer to this MSO question:
People have the same names in real life. Isn't [math.SE] real life?
In short, this is by design, as this MSO answer makes somewhat more clear. The unique identifying characteristic of a user is the user number; for me it is
8348. The "username" is more akin to a "display-name". (In the US this would similar to the difference between one's legal name (not necessarily unique) and one's SSN (unique).)
For the interested: 100 most common usernames on math.SE (data.SE query)