It’s possible to set up the patron registration form to either allow or disallow users to enter multiple email addresses for a single patron, separated by a comma.

To do this, go to Administration → Local Administration → Library Settings Editor. Search for the setting called ui.patron.edit.au.email.regex.

If you’d like to allow multiple email addresses, set this value to ^(?:(?:\b[^@,\s]+@[^@,\s]+\.[^@.,\s]+\b)(?:,\s?(?!$)|$))*$

If you’d like to disallow multiple email addresses, set this value to ^(?:\b[^@,\s]+@[^@,\s]+\.[^@.,\s]+\b)$