ESA and income tax

Hope someone can advise. Taxable ESA was paid to my client during 2011/12. She also had earnings in the same year so paid tax via PAYE. Revised tax notice shows she has tax to pay because of ESA. That would be correct.
However ESA was partially overpaid and she has now repaid this during 2012/13.

Should the tax for 2011/12 be revised being the year the ESA was paid, or the tax for 2012/13 be revised - the year the overpayment was repaid. She certainly shouldn’t pay tax on ESA that was repaid.

Client is currently trying to get proof of the amount overpaid and repaid to DWP and will be contacting HMRC, but I would be interested to know how this would work if anyone knows a little bit more about Income Tax ?
Thanks.