If you owed back taxes, the IRS would automatically seize your refund to pay the tax owed from a past year; how much did you owe? How much was your refund that was offset? If the refund was not sufficient to pay the amount you owed, that is why you are being billed for more interest.
What is shown in your online account with the IRS?
https://www.irs.gov/payments/your-online-account
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