Happened to me as well - six thousand duplicates from about 150 contacts. I managed to deal with it by resetting the Phone Link app in Win 11 on both my PCs, and then ignoring prompts to re-link my phone. Next I installed the Outlook Desktop email client (I have a MS365 subscription) on one of my PCs. The duplicates appeared in a folder named "<nameofphone> Synced by Phone Link". I highlighted the whole lot, deleted them, then deleted the "Synced by Phone Link" folder. After a while the deletions started syncing to my phone, and by the following day the duplicates had all gone. They also disappeared from the Outlook web client. My genuine contacts were untouched and the duplicates have not come back. There is no way I will re-enable Phone Link until MS acknowledge this bug and confirm it is fixed.
I did report the bug via the Feedback hub, and had a reply a couple of days later. I was obvious from the response that they had not read my original report properly. I replied again explaining exactly what had happened, and also the steps I had since taken to fix my problem. They said they were glad I had fixed and the conversation ended there. I did not have the impression that they were at all interested.