Let me just explain how it happens:
A = Client 1's query to change his name
B = Client 2's query to change his name
A and B arrive at master server, they must arrive at relatively the same time.
A checks for a pre-existing "Feep" in the database, finds out there is none.
B checks for pre-existing "Feep", also finds out there is none
A and B then proceed to change names, and the checks from before allow both to change names.
Notice how and A and B were both able to check before names were changed.
