I have been testing the paid Pro version automated WordPress User Registration function Add-On and have found a small programming error.
The validation for this form field checks to see if another User with the same username is already registered, BUT it differentiates between a username with Capitalization and non-capitalization.
BUT wordpress itself doesn't permit two users to share a username with only Capitalization differences, so I think this should be mirrored in the Plugin.
e.g. if existing user = Suzi , VFBp will report a validation error if new form user entry = "Susi". But VFBp will NOT report a validation error on the form if new form user entry = "susi" (and it should) ;-)