Underpaid tax for earlier years in your tax code for 2022-23 Collected Twice?

In 2021-2022, I underpaid tax. Normally if I underpay tax, I don't use the option for it to be collected via ther tax code. But, in 2021-2022, I did. I got a tax code notice to tell me (correctly) the amount of the underpayment and that the tax code would be adjusted to collect it during the 2022-2023 year. I've just done the 2022-2023 SA, and it has added the underpaid tax to this self assessment. I'm thinking "hold on, you already collected it via the tax code, so I already paid it - why ask for it again"? Two questions: 1. Is this normal? (If so, I won't bother ticking the box to adjust the tax code in future.) 2. If I do simply accept it and pay it, will it eventually all "come out in the wash", or is there a risk that if I don't fix it now, I might end up paying too much tax and never seeing the overpayment refunded. (I.e., can it "get lost in the system"?) If the answer to the latter is "no that's not possible", I'll pay it anyway as trying to get these things fixed over the phone is a potential nighmare!

Hi mossywell,
We need to include the underpayment collected in your code as this is part of the tax dedcuted at source by your employer.
You will get the credit for the tax paid when you declare this on your employment page so not paying the underpayment twice. 
Thank you.