Hello Shaw Tabrizi, 

Could you please tell me where exactly do you have this issue? Is it in the booking process, when you try to book as a customer? 

If yes, there are several options in Amelia for customers when you go to Amelia -> Settings -> Roles Settings -> Customer: 'Automatically create Amelia Customer user', ;Check customer's name for existing email when booking' and 'Allow customers to reschedule their own appointments'. So if you enable the first option the customers will be created automatically in Amelia and in WordPress as Amelia Customers, and if you enable the second option while they book Amelia will check their names if the email already exists. So, if you don't need this (if you don't want the email to be checked in the booking process) just disable this option and the issue will be gone. 

You can read more about these settings following this link and more on customers here.

If you have any further questions or concerns feel free to ask,