Hi,
This is not linked to JEvents or the integration between HikaShop and JEvents.
In the checkout, you can either:
- login with an existing Joomla account
- register a new account
- guest checkout with an email address which wasn't previously used to create a user account
There is no option or easy code modification to allow users to guest checkout with an email address already used for a user account. If we were to allow that, that would mean that by only knowing the email address of someone, you would be able to get the previous addresses they had used on the checkout due to the way the user handling is done.
To avoid that, it would require some complex modifications to allow customers to have both a guest account and a normal account with the same email address which would cause problems with some other functions of the checkout.
Now please note that this doesn't mean that you can't checkout as a guest several times on your website with the same email address. It just means that once you create a user account for an email address, you need to login with that email address instead of guest checkout.
If you don't want users to have user accounts so that they can always guest checkout, you can delete their user account in the Joomla user manager and they'll be able to guest checkout with their email address again.