Overpayment letter from the unemployment office - 6 weeks in

I received a letter saying I was overpaid about $1000 on my claim. I am in Baltimore, in case local advice matters. The letter mentions an appeal window and a repayment plan. 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.