Steps to reproduce the issue

• Fresh Joomla installation
• enable users registration
• To be more obvious, enable profile plugin
• go to frontend to registration form
• fill the form, use the same username, as is your administrator (or any other existing user)
• submit the form

Expected result

When displayed error on duplicate username, form should keep its values.

Actual result

Error is displayed, form is empty.

System information (as much as possible)

Joomla 3.7.2 fresh installation.

Additional comments

Seems there is problem in UsersModelRegistration->getData, there is getForm call, which causes recursion back to getData. As first line sets $this->data to empty stdClass, condition will not pass and lodFormData function receives empty values, so no data are loaded to the form.