Anthem Blue Cross Denying All Claims for having “Additional Insurance Coverage”

I am wondering if anyone has any insight into an issue my family is having. We have had the same health insurance with Anthem Blue Cross of California for about 5 years through my employer. For some reason, I noticed that all claims for my wife suddenly were getting denied throughout the entirety of 2023. Prior to that, never had issues with any claims. The reason code for denial was the below:

“Our records indicate that you have additional insurance coverage. Once the other insurance company finalizes the claim, please have your provider resubmit the claim with the other insurance company’s payment information listed.”

We do NOT have any additional health insurance from anywhere else. When I called them, they said we need to prove termination of benefits with the other insurance and that they would launch an investigation with their coordination of benefits team which would take 2 weeks. I don’t understand how we can prove termination when we don’t even know what the other supposed insurance is. Is this some kind of fatfinger typo that occurred by the insurance company? Is this a common occurrence and just a mistake that works itself out? We have no other insurance and no other employment that provides such benefits. I feel like this is an easy thing for them to figure out but they won’t do it (i.e., call up that company and confirm we don’t have a policy with them).

In the meantime, we are getting hounded by two billing departments asking us to fix this so they can get paid because their claims are being denied. Any help on this would be appreciated.