Got an overpayment notice, what now? after 14 years as a social media manager

The agency says I reported earnings incorrectly, but I think I followed the instructions. I would rather hear the honest version than the polite one. The letter mentions an appeal window and a repayment plan. 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.