[TYPO3-50-general] Validator: unique users

[Julian Kleinhans]

Hi list,

i have build a little restricted area
To create new users is no problem.. but, the problem is that i can 
register many users with the same username. I need a check that the 
username is unique. I cant find a validator for that, in the security 
package i cant find anything too. Is there a way available or must i 
code this myself ?

thanks and greetz
Julian kleinhans