My "solution" was to:
- Turn off contact sync in Link to Windows on android
- Turn off the Your Phone app (unlink the phone)
- Using the Outlook for Microsoft 365 app (comes with paid business version of a subscription to Office 365), adding the outlook.com account to it, letting it sync the contacts and then deleting from that app, in the Contacts-section, deleting all the duplicate contacts (located in a separate folder called Link to Windows) and letting it sync for an hour or so. All the duplicates vanished from the desktop app, but about 8 000 were left in the cloud,
- The following day, I added my outlook.com account to the Outlook for Microsoft 365 app I have at my office, and again synced, deleted and waited. Now the cloud duplicate contacts were down to about 1 600.
- Then on the outlook.com website I deleted 10 contacts at a time (a slow and tedious process, where you have to wait 30-60 seconds for the delete-option to become available after selection). It took about 3 hours of my life.
I will never, ever use Link to Windows again, and I from now on I will sync my SMS via the google solution.
It is astonishing that Microsoft quality control has failed so totally, and also unbelievable that you cannot delete more than 10 contacts at a time in the cloud.