MacOS – Contacts.app: How to merge duplicate contacts

I fixed this by manually setting the "middle name" to "A" on all Outlook contacts, then synced with the iPhone, then manually deleted all contacts without an "A" middle name on the iPhone, then removed the "A" middle names in Outlook and then synced with the iPhone again.

Drawbacks: lots of manual work and loosing the middle name content (luckily not used in my contacts).

Using the middle name is good because it is shown in list view on the iPhone, so one can quickly see what must be removed without looking at details.

The best procedure is to one by one, turn off each cloud sync source of contacts.

They will remain in the cloud and be removed one by one from your iPhone.

When you have all cloud accounts turned off as far as injecting contacts into the contact app, you will be left with only the local groups and contacts stored on the device itself. If you don't wish to have a local contact on the phone, sync and delete these from a computer (or manually from the device).

At that point, re-establish the cloud sync for each account - checking that there are no duplicates after each addition.

Once you have eliminated the possibility of duplicates locally vs. cloud - you can quite easily clean up the cloud source of duplicates if needed.