Got an overpayment notice, what now? (update)

After 23 weeks of benefits I got a notice saying I must repay some of it. This is my first time dealing with any of this. I did not notice any errors at the time. Has anyone disputed one of these?

Thanks for writing this out. I went through something close to it 10 months ago, and it got easier once I kept a simple log of every call and form.