iCloud produces dozens of copies of my address book entries
I've been using several Apple products for quite a while now, most of them without any problems. But two or three month ago my Apple address book/contacts started to double again and again and again. I've already deleted the address book on each and every device and inside the cloud in order to rewrite every single entry - which was quite a bit of work, since I have had some hundred entries to reenter.
Unfortunately, the problem still appears. It seems, that with each new addition or change, the address book makes a new copy of the group, in which I have put that entry. Another problem: Quotation marks are being added as well, which change the order of my groups within the address book.
I've already read within a German online community, that there might be a sync problem on Apple's part. Seemingly, Apple does not take any effort to find a solution.
Hopefully, someone here can give me a good advice.
(All my devices - iPhone 6, MacBook Pro 2015, iPad Mini 2 - have the latest software versions.
MacBook Pro with Retina display, OS X El Capitan (10.11.3)