Repeated Bills (and Threats To Send To Collections Agency) For Something I've Paid

My situation is kind of absurd. I went to a walk-in clinic in late November and they ended up sending a test to Labcorp. My insurance paid for part of that test, but I was left with a $25 bill. I wasn't really paying attention and paid that $25 at the start of December, via the UHC website (rather than directly to Labcorp). UHC website states that Labcorp accepted payment on 01/02/25. Since that date, I have received several bills from Labcorp (by email, text message and post). The most recent bill is a"collection notice" and they are threatening to send this to collections and damage my credit rating.

Weeks ago, I wrote to Labcorp and told them I had paid via the UHC website, and asked if they were aware of the fact that this is something that can happen. I also provided them with the reference numbers for the payment that UHC provided me with. But they do not respond to my email messages beyond saying "please call billing". I've spent over 30 minutes on the phone waiting to speak to a human, before either the call just drops, or I have to do something else.

I'm not going to pay this $25 that I have already paid, and I feel that Labcorp are harassing me and trying to extort me, and will potentially harm my credit score. (or it's possible UHC are lying and haven't actually paid them?! lol?).

Any ideas for how to proceed here? Do I have a case against them for harassment as it is?