Hi,
Once a customer creates a Joomla user account, be it via the Joomla registration or the HikaShop registration, he can't register againt with the same email address. And we can't allow him to do a guest checkout with an email address from a Joomla user account since otherwise, he would have access to the information of the user account without being properly logged in.
In that case, he should log with his user account.
However, if the customer didn't register, and only did a guest checkout, he will be able to do a guest checkout again with the same email address, and register if he wishes.