The IRS has not addressed this yet. They will not open until February 12 and at that time they will start to see situations like yours. You would not be able to amend until that time and in the meantime, they may establish a guideline.
Information is continuing to be updated. Please see this LINK for more information.
It is best to not spend the check as they may request it back.
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The IRS posted that they will be checking all the stimulus entries on the returns against their records and if your return is incorrect the will fix it automatically however this could increase your processing time. Just sit tight and wait.
question for you 100 percent of the time my taxes are seized for child support so this 600 would be the fist money I would be getting back in a long time so if it does not matter about extending processing time would it be feasible to cash the Check and let them correct the 600 dollars on the taxes
I highly doubt you will be getting this $600 either ... only the ADVANCE payment was exempt from child support but the amount added to your tax refund is up for grabs. I have not seen anything posted that contradicts this information.