Apple Contacts duplicates individual contacts and groups

To resolve the issue of Apple Contacts duplicating individual contacts and groups on your M3 iMac (macOS Tahoe 26.6.2) and iPhone 13 mini (iOS 27.0.1), you can follow a two-part approach: merge individual contacts using Apple's built-in cleanup tools, and fix the underlying iCloud sync glitch responsible for duplicating your contact groups. 

Clean Up Individual Contacts

Apple provides automated tools on both macOS and iOS to sweep through and combine duplicate entries.

On your Mac

• Open the Contacts app.
• Click on the Card menu at the top of your screen.
• Select Look for Duplicates.
• Click Merge when prompted. Repeat this step until the app confirms no more duplicates are found.

On your iPhone:

• Open the Contacts app.
• Look directly below My Card at the very top of the list.
• If duplicates are detected, tap View Duplicates.
• Tap Merge All to clean them up instantly.

More info from Apple: If you see duplicate contacts after setting up iCloud Contacts - Apple Support

Reset iCloud Sync to Prevent Recurrence

If the duplicates keep coming back, the local database sync loop between Tahoe and iOS needs a refresh.

• On your iMac: Go to System Settings > [Your Name / Apple ID] > iCloud. Toggle Contacts off. When prompted, choose to delete the local contacts from your Mac (they will remain safe in iCloud). 
• On your iPhone: Go to Settings > [Your Name] > iCloud > Show All. Toggle Contacts off and choose to delete them from your iPhone. 
• Reboot both devices to clear any stuck system cache files.
• Go back into your iCloud settings on both your Mac and iPhone and turn Contacts back on. Give it a few minutes to fetch a single, clean copy from the server

Check for Secondary Account Conflicts

If the duplicates return after a few days, check if a non-iCloud account (like Google, Outlook, or Yahoo) is syncing contacts simultaneously.

• On iPhone: Go to Settings > Apps > Contacts > Accounts.
• On Mac: Go to System Settings > Internet Accounts.
• If you have contacts turned on for Gmail or Outlook alongside iCloud, your devices will pull from both sources, forcing them to duplicate on your screen. Turn off contact syncing for secondary accounts to keep a clean, singular list

Also, note that the current version of Tahoe is 26.7.2 (and your Mac can run Golden Gate, which is on 27.2); the current version of iOS is 27.2.

Contacts menu/Card/Look for Duplicates. You have the option to merge the duplicates.

Quick Easy.

Regards

I cleaned up my Duplicates on my iMac, and the changes were also reflected on my iPhone. Do I stop there and see if they reappear, or continue on to part two of your reply? - Paul

I'd just leave it be, unless the duplicates show up again.