Overpayment letter from the unemployment office, bookkeeper here

After 6 weeks of benefits I got a notice saying I must repay some of it. Any experience at all would help. I did not notice any errors at the time. Who should I talk to first?

Thanks for writing this out. I went through something close to it 12 months ago, and it got easier once I kept a simple log of every call and form.

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