My first Universal Credit payment is wrong, can I have advice please?

Thanks for your advice. Surprisingly I have another issue.

On a post above I mentioned I'd received a letter about an over payment of about £900. I was overpaid income related ESA as my application took a while as I had my ID check at home which was postponed, and the date of my application too I think.

The letter from DWP, Citizens Advice, and a couple of websites all said the money would be taken back in instalments capped at about £60, which would have taken 15 months to repay. I don't like being in debt so I paid the money back to DWP following the instructions on the letter, including using my NI number as a reference. I thought the matter was resolved. However, when my second payment arrived they had deducted the £900, so I've now paid it back twice.

I'm a bit fed up with the whole process now, if I just wait will the money come back to me as an underpayment? It seems it should. If I need to appeal, what's the process please?

Thank you!