Got an overpayment notice, what now? after 15 years as a line cook

After 18 weeks of benefits I got a notice saying I must repay some of it. I have read a lot of threads here and could not find my exact situation. I did not notice any errors at the time. Has anyone disputed one of these?

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