Why someone was able to take the same name as mine on the History stack exchange? I assumed that the software should prohibit this automatically.
As I understand it, the site maintains a unique user ID for every user (yours is user:3353 and mine is user:24858), but the username does not have to be unique.
The profile picture can help to distinguish between users with the same (or very similar user names).
The issue has been raised and discussed at some length on SE:meta.