You will need to file an amended return (and pay back the excess received) by changing the amount of withholding to be correct in the return and entering the explanation, etc. the first column of the 1040X should then show the amount from the original return, the 3rd column to be the actual amount withheld and the middle column should show the amount of difference in the withholding.
Oh yes, IRS will correct the error and you will have to repay the excess received and should repay it with the amended return filed..
