When e-filed 1040, had not received 2nd stimulus - indicated that on return. Now received stimulus before return is processed. 1040X not ready b4 IRS processes. Now what?

If you claimed the Recovery Rebate credit on your 2020 tax return because you did not receive the second stimulus then subsequently receive your second stimulus payment by mail or direct deposit, I advise that you should wait to receive your refund before taking any action.

It is likely that the IRS will cross-check with their records and adjust your refund. If they do, all is fine and nothing needs to be done. If they do not adjust your refund, then you should amend your return and correct the Recovery rebate credit. With your amended return, you will enclose a check equal to the overpayment.

If you owe taxes, I advise that you make an additional tax payment before April 15, 2021 equal to the overpayment. You can pay online at irs.gov/payments and apply the payment to your 2020 form 1040.