User Registration + Facebook Integration--> Missing validation

I have one issue with kentico 9 which is normal registration using CustomRegistrationForm webpart, and FacebookConnectLogon webpart.

The scenario is: If user already register using CustomRegistrationForm webpart, then the same user click on the FacebookConnectLogon webpart. In database both user are register, one with facebook id, one with user name/email .. At this stage, this both user cannot edit via cmsdesk because both will throw error "user with email adready exist". I have send support ticket about this issue.

Below are the respond from kentico support team.

"I have update for you from our Product Owners:

I'm really sorry, but this scenario is not supported. When user has been created via registration form, he shouldn't use external login (Facebook) and vice versa. It is not possible to mix internal and external login on single user. Also having multiple users (one for Facebook login and one for normal login) for single real person is not best practice."

So guys, please remember on this situation. How on earth normal user will know that he/she already register via normal form and should not click on facebook login button. Does not make any sense.