Got an overpayment notice, what now? - first time posting

After 21 weeks of benefits I got a notice saying I must repay some of it. I do not usually post, so bear with me. 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 2 months ago, and it got easier once I kept a simple log of every call and form.