Insert the same time, it is possible to happen?
Theoretically you could end up with duplicate usernames. 
In a scenario of high incoming requests, a race condition could occur. 

If you want to avoid this, just put an UNIQUE index on your column. 
You could create a UNIQUE index on username by using this query:

CREATE UNIQUE INDEX IDX_USERNAME ON `user_tbl` (`username`);


Prior to creating this unique index, please verify that there are no duplicate entries in your table, as the index creation would fail. 

When you have an unique index on this field, it will be impossible to end up with duplicate usernames. 
When a username is inserted, the table is locked during the insertion, this guarantees that a race condition would not occur.