Can the same email be used for a guest checkout user over and over?

When the guest checkout is completed, a password is sent to the guest user. However, until the guest actually logs in using that password, the user account is still marked as a guest. Once the account is converted to a "real" account by the user logging in with the email address they used and a password, they will no longer be able to checkout as a guest with that email address again. If they try and use this same email again later as a guest they will receive a message that tells them that the email address is already in the system. At that time they can either locate the credentials sent to them or reset the password and continue. At this point, they will no longer be considered a guest and both the current order and the past order will be available for them to view in the site. 
For more details on Guest Checkout, please refer to this article: Ch. 028. Guest Checkout