Massive number of duplicates on contacts.

If the problem is a corrupted address book, and there are no contacts stored "On Mac", then I would recommend:

1. Close the Contacts application
2. In Finder, click the "Go" menu, then "Go to folder..."
3. Enter "~/Library/Application Support/" in the box and hit return to visit that directory
4. Drag the directory "Address Book" to the trash
5. Open Contacts application and wait a minute for contacts data to repopulate

That would be far less extreme than erasing and reinstalling macOS for an issue with the data of one application.

Hopefully what you are seeing is some type of local corruption and not real. Try this:

1. Back up your Mac with Time Machine - Apple Support
2. Remove the iCloud account from the Mac: Remove or temporarily turn off email accounts in Mail on Mac - Apple Support
3. Restart the Mac: Shut down or restart your Mac - Apple Support
4. Login to iCloud.com using a web browser and inspect the contacts as they are listed on the server. Troubleshoot the issue, merge, delete, etc. there if necessary.
5. Add the iCloud account back to the Mac: Add email accounts in Mail on Mac - Apple Support
6. Check your contacts

The Mac address book is corrupted , for normal working Mac few duplicate contacts might be created but not in thousands , erase the hard drive and reinstall Mac OS .