That’s an interesting observation!
Usernames are effectively case-insensitive, so you shouldn’t be able to have two users with the same username. However:
You can see what appears to be two distinct usernames returned when searching for fatemeh12. This shouldn’t have been able to happen (I’m not able to reproduce it). Both go to the same profile it seems.
I’m at a loss on where to go from here. If someone doesn’t chime in by around this time tomorrow, I’ll see if I can track someone down with some fresh ideas. If you find any additional information that you think is relevant in the mean time, please post it.