Overpayment letter from the unemployment office - any advice?

After 15 weeks of benefits I got a notice saying I must repay some of it. I have been trying to stay organised about it. I did not notice any errors at the time.

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

Keep everything in writing and save the confirmation numbers. It made the follow-up calls so much easier.