Got an overpayment notice, what now? - what worked for you?

After 21 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. Who should I talk to first?

Thanks for writing this out. I went through something close to it 5 months ago, and it settled down once I kept a simple log of every call and form.