Just circling back with some more information for anyone who may encounter this problem. I finally got through on the phone line this afternoon. In fact, I had to wait on hold for over 30 minutes two separate times because the first time I was disconnected trying to be transferred to the Integrity Team at the DUA.

I was told that the system sending out these "overpayment" letters does not take into account whether the person has a fraud report on file (identify theft). The second agent I spoke to said that she's getting tons of calls from people like me. She gave me the Integrity Team phone number and advised me to contact them. She said that they've been cleaning this up for many people. They did not answer this afternoon, but I'll keep trying tomorrow.

Separately I also mailed an appeal letter to DUA (certified mail), and attached my 2020 fraud report (receipt) and copies of the letters, and explained the situation. I'm feeling optimistic that at least one of these channels will work!