I filed taxes early and put that I did not get stimulus, however a check ended up coming in the mail. What do I do now as amending my return will not work?

If you have e-filed your 2020 Federal tax return and the tax return is rejected by the IRS, you will have the opportunity to correct the entries and calculations to generate the Recovery Rebate Credit as well as correct the entries that generated the rejection.

If you have e-filed your 2020 Federal tax return and the tax return is accepted by the IRS, we will see what approach the IRS takes in your situation.

I suspect that the IRS will scrutinize stimulus payments that were issued against the tax returns that claim the Recovery Rebate Credit.  The IRS may be able to correct your tax return and your refund much like it corrects tax returns and refunds which misstate Federal withholding tax.

Follow the TurboTax COVID Tax Center and the IRS Economic Impact Payment Information Center for further developments.