Underpaid tax for earlier years

Hello, My accountant submitted my self assessment for 23/24 and confirmed a tax bill of £296.40. I have since received a letter from HMRC to say that they do not agree with the calculation and have made changes. HMRC is saying that underpaid tax for earlier years in my tax code for 23/24 is £1000.80 and my tax bill is therefore £1,297.20. Surely if my tax code in 23/24 had been amended to recover underpaid tax for earlier years, I have already paid this now and do not need to pay it again with my self assessment? Thanks.

Hi nath43,
To review this you will need to contact HMRC to access your records https://www.gov.uk/government/organisations/hm-revenue-customs/contact/self-assessment
Thank you.