Incorrect username on sign up

I am having trouble in changing the error message if a user tries to sign up with a username that already exists. It currently says incorrect username instead of username already exists,

I have been able to set it up for email by following the docs but I cant find anything regarding the username.

when I go to registration form edit under username edit I cant even select custom validation is there any way to resolve the error message to display username already exists.

Regards