Erroneous Tax Owed From Previous Years ?

I have just completed my 23/24 online Tax Return - I repay Child Benefit via the system. However, the calculation of tax owed includes 'tax owed from previous years', specifically the Child Benefit total from 22/23. As far as I am aware, this has been collected during the 23/24 tax year via PAYE as I do each year. The Notice of Coding for 23/24 includes this amount, my Tax Code was adjusted to recover the money each month, and my payslips for this period all reference that code. Any idea why the system still thinks I owe this ?

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