@Aendil No one might ever know how you got 2900 on line 30. But you know you already got both the first 2 stimulus checks so line 30 should have been blank. The IRS knows they already sent it to you but you tried to claim it again on your tax return. There's nothing you can do about it now. Did you get the right amount for the 3rd stimulus in 2021? Make sure you don't try to claim it again. Better check your return over close before you file next time.
Did you get the full $4,600 for the first 2 EIP? How old were your dependents?
