Best tool to cleanup disorganized contact list with duplicates?

[logicstudiouser]

I was going to help a friend of a friend fix their business contact list full of duplicates in iCloud. But from my understanding, there is no official way to do this from Apple.

What would you recommend to do this efficiently? My concern with third party apps is privacy as these are business contacts.

 

[Weaselboy]

You could use the built in feature of macOS Contacts for this.