Got an overpayment notice, what now? after 10 years as an IT help-desk technician

The agency says I reported earnings incorrectly, but I think I followed the instructions. 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.

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