If you have bank and/or credit card statements showing the amount of tax you paid then follow the instructions on the CP23 notice to contact the IRS. The letter contains an 800 number for you to use.
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Check your 1040 line 26 for estimated payments and amount applied from 2023. I have seen people who somehow got the amount applied from 2023 in twice. You need bank statements showing all the estimated payments you made to backup line 26. And check your 2023 return line 36 for the amount applied to 2024.