This hook controls whether or not to allow registrations with email addresses which have already been used. By default, PMPro does not allow multiple registrations with one email address and shows an error if the email address has already been used.

$oldemail = apply_filters( "pmpro_checkout_oldemail", ( false !== $oldem_user ? $oldem_user->user_email : null ) );

Parameters

$oldemail

string

Source

File: https://github.com/strangerstudios/paid-memberships-pro/blob/master/preheaders/checkout.php