Got an overpayment notice, what now? after 10 years as a web developer

I received a letter saying I was overpaid about $1050 on my claim. I have been trying to stay organised about 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.