How to disable email verification on guest checkout

[Levi]

Hi, i want to disable the email verification on the checkout form.

Now if a user wants to create an order as guest, but the email is already registered, display the error `This email already registered...`. 
Where i can disable this, which controller? 
I want to make it possible the guest checkout, even if the email address is already registered.

PS. 1.7

Thanks for answers

Edited September 28, 2018 by Levi (see edit history)