Having to pay back health insurance for months I was covered by an employer sponsored health insurance.

Apparently there was a covered california insurance policy open for myself, during the same months I was getting employee-sponsored health insurance.  Now they're requiring me to pay back thousands for the "premium tax credits" they "gave me".  I even have a 1095-C form which my employer gave me showing that I had insurance through them.   Does anyone have any advice?  I'm paniking -_-.