Synchronize then delete contacts in "On My Mac" with iCloud

Again, many thanks. I have been able to verify my suspicions as to the origins of On My Mac. It seems that, anytime one turns off the use of iCloud on a device, not only on a Mac, one is asked whether one wants to make a local copy. This happened when when I turned off the use of iCloud on my iPhone. I agreed to a local copy, and All iPhone appeared in the the sidebar of the Contacts.app. I deleted everything in it (while iCloud was still off in it), and All iPhone also disappeared. Reconnecting Contacts.app to iCloud brought back the contacts in the iCloud, and there was still no All iPhone in the sidebar.

So I assume whatever is in All on My Mac is obsolete. I have never used the Mac for long without a reliable connection to the Internet, the only time that I could imagine adding to, deleting from or modifying the contacts in All on My Mac. And now that I know about this behavior, in the future will be careful to denote a new or modified card, by, say, appending *new* to the last name, so that I could move just these into the iCloud, delete the obsolete ones and rename the *new* ones, thereby leaving nothing to chance, resp. to a disagreement between how I believe merge should work and how it really does. After the manual merge, I would delete All on My Mac.

So I consider this question solved and thank you for your valuable input. Whether one wants to delete All on My Mac or move those contacts into iCloud and merge really depends on how much damage the merge would cause and how much weeping and gnashing of teeth manually repairing the damage would entail. In my case, I chose to just delete All on My Mac. In a few cases I might have made modifications to cards in All on My Mac without mirroring the same modifications in All iCloud, but I doubt it, as I would have noticed the discrepancies when viewing the affected contacts on the iPhone or the iPad and resolved them.

Thanks, very helpful. I found how to recognize duplicates before performing the move. I select all the contacts in All on My Mac, then click on All Contacts. The selected contacts are displayed with a background that denotes that they have been selected, while others are displayed without a background color.

Perhaps you know: When deduplicating after the merge, will I still be pestered if the cards are exactly the same?

I still hesitate. Let me tell you why.

First, in All on My Mac I have 477 contacts; in All iCloud, 553. In the majority of cases, if not all, the contact in All iCloud should be kept, but I would appreciate more information than what Cancel-or-Merge prompt in your reply offers. A side-by-side comparison would be more helpful.

Second, the real problem that I'm trying to solve is this: birthdays in contacts are appearing twice in Calendar.app. I want them to appear just once. It seems that deleting All on My Mac would solve this problem, but not knowing how All on My Mac got there in the first place, and whether something is maintaining it (sort of like a cache or backup), or whether, once I delete all the contacts there nothing will magically appear there, makes me reluctant to take the dive. Except for the duplicate birthdays in Calendar.app nothing else on the Mac seems to be affected.

Third, a second MacBook Pro has the same problem, except that its All on My Mac has two contacts more.

If you, or anyone else, can shed more light on those issues, I would appreciate it. Otherwise, I suppose I'm just going to have to take a timid first step and, say move just a few contacts to All iCloud and see what happens.