Can you clarify your question please?
It sounds like you are amending your federal return to show that you only received $600 in stimulus payments instead of $1,200? If that is correct, you will need to review your original return as filed to ensure the Recovery Rebate Credit was not already calculated on your return as originally filed.
You will need to look at your Form 1040. If you claimed the Recovery Rebate credit, it will show up on Page 2, line 30 of your Form 1040 as the Recovery rebate credit. See the sample form below.
If there is an amount on line 30, do not file an amended return. The IRS will be reviewing the returns that calculated this credit and will make any necessary corrections.
If there is not an amount on line 30, go ahead and file the amended return to claim the credit.
As for California, it is hard to tell what happened there as we cannot see your tax return since this is a public forum. I would recommend reviewing your return as originally filed to determine if you can see if anything else changed. If your only reason for amending was for the federal Recovery Rebate Credit, you should not need to amend your state return as the credit would not have affected any of the amounts that flow in from your federal return.
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