Got an overpayment notice, what now? - any advice?

After 18 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.

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