Overpayment letter from the unemployment office - 13 months and counting

After 24 weeks of benefits I got a notice saying I must repay some of it. I did not notice any errors at the time. Who should I talk to first?

Collect your pay stubs and certification records for the weeks they name. Mine showed the error was a date mix-up.

I asked for a waiver and a plan in the same call, and they gave me both forms. It was less scary than I expected.