Fix OpenCart Checkout Issue

Hello,

I need your help fixing a checkout issue on my website.

The store is running on OpenCart 3.0.3.6.

Problem:
When a customer tries to place an order from the cart/checkout page and enters an email address that has already been used for registration before, the checkout does not allow them to continue. Instead, it shows a message that the email address is already registered.

Because of this, returning customers cannot place a new order unless this is handled properly.

What needs to be fixed:
Please check the checkout flow and make sure that:

A customer who already has an account can still complete a new purchase without being blocked by the “email already registered” message.
The checkout correctly handles existing customers, for example by allowing login, guest checkout, or another proper flow.
The system should not treat every checkout attempt as a new registration.
If needed, adjust the checkout logic, theme, or any checkout extension/module that may be causing this behavior.
The solution should work smoothly for both new customers and existing registered customers.

Important:
I do not just want the error hidden. I want the actual checkout logic fixed properly so returning customers can order again without problems.

Please investigate whether this is caused by:

OpenCart checkout settings
the theme
a one-page / quick checkout module
or custom checkout code

Then apply the proper fix and test it.

Thank you.