UserName exist in database or not?

Hi All,
        How to check wether username exist in database or not in my regestration page?
and plus i want to implement this feature also...suppose i allowed one user to select username as  'user01' and that user is still completing his form and same time another user tries to check whether 'user01' exist in database or not then i should show user as that 'user01' is not available for him and he should try to select some other username for him...
How to handle this???


Thanks..

  



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