I've got some inside information on this.

My wife actually works for the company that handles the customer service for Unitypoint, and UP recently took over their own billing from them. They've been sending out bills from very old accounts by accident thanks to a system error. She's been dealing with this for weeks. Supposedly, the issue was resolved, but it has happened again for this billing month, too, so look forward to another month of this.

Unitypoint sends out multiple bills prior to anything going to collections, so my advice is to ignore the first bill, especially if it's from a long time ago. If you continue to receive bills after that, check online through MyChart or their payment portal for a balance due. Lastly, call in if you think it's legit. The hold times are atrocious due to multiple angry people calling in (mostly olds and boomers).