multiple N properties are forbidden and not repaired

Steps to reproduce

1. share a contact from user 1 to user 2
2. switch to user 2
3. try to change the "group" via user 2 to a contact entry from user 1

as requested from:
https://help.nextcloud.com/t/contacts-2-0-1-cant-change-group-on-shared-contacts/21870

Expected behaviour

contact get assigned to group

Actual behaviour

Sabre\DAV\Exception\UnsupportedMediaType: Validation error in vCard: N MUST NOT appear more than once in a VCARD component

/var/www/owncloud/3rdparty/sabre/dav/lib/CardDAV/Plugin.php - line 294: Sabre\CardDAV\Plugin->validateVCard('BEGIN VCARD\r\nVE...', false)
[internal function] Sabre\CardDAV\Plugin->beforeWriteContent('addressbooks/us...', Object(Sabre\CardDAV\Card), 'BEGIN VCARD\r\nVE...', false)
/var/www/owncloud/3rdparty/sabre/event/lib/EventEmitterTrait.php - line 105: call_user_func_array(Array, Array)
/var/www/owncloud/3rdparty/sabre/dav/lib/DAV/Server.php - line 1128: Sabre\Event\EventEmitter->emit('beforeWriteCont...', Array)
/var/www/owncloud/3rdparty/sabre/dav/lib/DAV/CorePlugin.php - line 513: Sabre\DAV\Server->updateFile('addressbooks/us...', 'BEGIN VCARD\r\nVE...', NULL)
[internal function] Sabre\DAV\CorePlugin->httpPut(Object(Sabre\HTTP\Request), Object(Sabre\HTTP\Response))
/var/www/owncloud/3rdparty/sabre/event/lib/EventEmitterTrait.php - line 105: call_user_func_array(Array, Array)
/var/www/owncloud/3rdparty/sabre/dav/lib/DAV/Server.php - line 479: Sabre\Event\EventEmitter->emit('method PUT', Array)
/var/www/owncloud/3rdparty/sabre/dav/lib/DAV/Server.php - line 254: Sabre\DAV\Server->invokeMethod(Object(Sabre\HTTP\Request), Object(Sabre\HTTP\Response))
/var/www/owncloud/apps/dav/lib/Server.php - line 258: Sabre\DAV\Server->exec()
/var/www/owncloud/apps/dav/appinfo/v2/remote.php - line 33: OCA\DAV\Server->exec()
/var/www/owncloud/remote.php - line 162: require_once('/var/www/ownclo...')
{main}

Server configuration

nextcoud 12.0.3 wioth contact 2.0.1

Operating system:
ubuntu 16.04
Web server:
apache 2.4
Database:
mariadb 10
PHP version:
7.0
Nextcloud version: (see Nextcloud admin page)
12.0.3
Contacts version: (see Nextcloud apps page)
2.0.1
Updated from an older Nextcloud or fresh install:
updated
Signing status:
official via appstore

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Start by reproducing the shared-contact group update described in the steps, then trace the CardDAV validation path from Sabre\DAV\Server::updateFile and Sabre\CardDAV\Plugin::validateVCard in the reported stack trace. Done means the contact can be assigned to a group without the duplicate N validation error.