ZenCart you cannot have two customers having the same email address.
True..

Past COWOA used a method of placing a dot in-front of the email address to hid it from ZC. around 2.7 this was changed to a column setting to ID the account type and behave accordingly. Upgrading is the best fix, but may also leave a mess in user accounts so a follow up with phpmyadmin to correct database issues would also be advised.