How to see Contact Duplicates in Mac OS El Captian
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.