iCloud Contacts app reverts changes and duplicates

Are you sure you are not syncing Contacts from another source other than iCloud? Check in the Contacts app by going down to Setting and choose the Accounts tab. If you have other accounts there Enabled for contacts syncing, then compare your results with those unchecked.

Ok, two additional questions:

• Are you using another device with the same Apple Account? If so, do you experience the same on that device?
• Are you using a VPN, Firewall, or Antivirus on your Mac?

What you describe sounds exactly like a syncing issue where the change on your device is not sent to the server and reverts back when the server syncs back to the device. A VPN will block outbound connections along with those other identified apps. The best way to determine if there are any of those apps installed on your computer is by posting the free EtreCheck report using the Additional Text option when posting.. It contains no personal information.

How to use the Add Text Feature When Post… - Apple Community