Forminator: Prevent multiple submissions from same email

add_filter('forminator_custom_form_submit_errors', function( $submit_errors, $form_id, $field_data_array ) {
    global $wpdb;

    // Retrieve the email value from the form field
    $form_email = $field_data_array[1]['value'];

    $sql = $wpdb->prepare(
        "SELECT COUNT(*)
        FROM {$wpdb->prefix}frmt_form_entry_meta
        WHERE meta_value = %s",
        $form_email
    );

    $results = $wpdb->get_results($sql);

    if (!empty($results) && $results[0]->{'COUNT(*)'} > 0) {
        $submit_errors[]['email-1'] = __( 'The email provided already exists.' );
        return $submit_errors;
    }
}, 10, 3 );

https://medium.com/@akshayharshan97/enhancing-forminator-custom-form-validation-with-the-forminator-custom-form-submit-errors-filter-35b2ede40437