Skip Registration for Users who are already registered

We have a sales team that makes outbound calls, and makes sales over the phone. As part of the process, we gather the customer’s email and input that email for the sale. Once the form is submitted, we use the “Registration-Add On” to register the user and send the customer an email to set/create a password.
As our sales have grown, we occasionally have the same customers buy more product over the phone. When we input their email address for the sale, and try to submit, the form won’t allow-us to submit because the customer is already registered on the website. Is there any code that I can use to by-pass registration for customers that are already registered on our website. I have seen this request many times on different blogs for the last 9 years and have not seen a proposed solution that works. I don’t want to have our sales team select a checkbox for conditional registration. Is there any code that can be added into my functions.php code that would bypass registration if the email already exists?

Sure, how about this approach from Gravity Wiz:

Hi Chris, this seems to be to skip registration for users who are logged in. That is not what we are looking for. We are looking to stop registration for an email that is already registered on the website. That doesn’t mean that they will be logged in when submitting the form

Hi Mikhail. Thank you for the clarification. In that case, I think this code fills the need: