You would have to amend the return for the year when the original excess contribution was made and every year after that during which the excess contribution was not corrected. Since you made the correction distribution in 2020, for the years of 2016 to 2019 you need to find out if you maxed out your pension plan contributions, and if so then you need to pay an excise tax on the excess contribution that was in the retirement account each of those years.
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