Multiple contact entries with Outlook 2016

Hello,

I have configured Outlook 2016 to sync with Zextras Mobile. Syncing mails and calender entries works (but takes a lof of iterations until the last month is fully synced).
But after some sync iterations I get multiple contact entries in Outlook 2016 and the sync status (in Outlooks bottom right corner) keeps on "syncning contacts".

I have already configured an Accountlogger for debugging. What should I look for?

Zimbra 8.6.0
Zextras 2.2.4

Thanks!

Danny