If you have already reported the over-contribution in 2020, then you do not report it again in 2021 even if you receive a form 1099-R. In 2021, you should only report any earnings on the over-contribution if applicable. You do not have to amend your 2020 return.
Pages 10 and 11 of IRS Pub 525 under Excess deferrals (the IRS term for 401K contribution is deferral) tells us to include the excess deferrals as income on line 1 of Form 1040 if the money was returned after the end of the tax year but by April 15th of the following tax year. You need to report only the excess contribution, not any money generated by the investment of the excess contribution. What you earned will be covered by a 1099-R for the following tax year and will be entered then as a normal 1099-R.
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