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I have a question I can't find an answer to.

If income in 2019 is overstated because in 2020 the TP had to repay it - why can't 2019 be amended to reduce the income in that year? I'm looking at helping a person who included income on their 2019 return - paid say $4,000 in taxes on it.  Now 2020 comes along and he has to repay half of it. Would the TP get a refundable credit of $2,000 on the 2020 return? The 2020 return shows only retirement income and social security so when the standard deduction is put in the TP has zero income and zero taxes. Thanks!