Error: Can not create a new user as the name is already in use

Hi, I have encountered a strange problem. As you can see in the image I am trying to create a new user account. The problem is even though there is no user by the name HDO (there was one but I deleted it), it says the user name is already in use. Can anyone tell me why it is behaving like this? and how can I solve this issue? Thanks  
1 answers

You probably still have a System.User entity with a Name attribute of that value. That is where the validation message is coming from.