If you haven't received your stimulus payment by the time you file your tax return, you should indicate that when you file your tax return. If it was sent out and you receive it before your tax return is processed, the IRS will just deduct the stimulus payment from your refund, since they know how much they sent you.
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I suggest you wait until at least the 25th to file the return indicating the missing money since the IRS had until 1/15 to mail out the payment. And since the IRS will not process anything until mid February you have time to wait and be patient. If you get the money after you file then the IRS will delay your return as they correct the error ... no one needs to file in a rush this year.