Hi,
if someone who has already bought something comes to the page and enters their email address, the cart will not allow them to checkout
Do you have any error message display when the checkout refuse the email address ?
HikaShop guest checkout does not allow to use emails which already have an Joomla user account.
It means that if you just use the email for guest checkout, you should be able to re-use afterwards.
But if you create a Joomla account with the email, HikaShop won't allow you to use this email because you can "login" with.
Regards,