How to see Contact Duplicates in Mac OS El Captian I am using the Mac OS Contacts app version 9.0 (latest).
I only have the option to "Look for duplicates..." which will then allow an auto-merge option if it finds duplicate. What it doesn't do is show me which entries will be merged. 

Other Apple apps and the old contacts app allowed showing duplicates. This feature seems to have been removed in the last 3 (?) major Mac OS releases. Mac OS 10.6 had it.

How can I find out what will be merged before using the auto-merge option?
I don't think there's a window to approve changes anymore (Contacts v9 on OS X 10.11), so you would need to parse a contact export or track the changes with a different tool than the contacts app. I just export the contacts database to a file on the desktop. Do the merge and see if it's what I wanted. 

If not, restore from the backup or undo the merge.

The only thing to be aware of is checking whether you have zero, one or more than one cloud backed contact syncs - when the same contact is in two containers - the merging combines the data but preserves the each entry in each sync class.

Also, I make sure I'm connected to the internet when I make (potentially) large changes so that the contacts app can accurately feed my changes to the cloud sync.