I am in the same situation, but the answer that a previous year return has to be amended is not correct in at least my situation, according to my accountant from last year and IRS pubs. Tax laws changed in 2018, and in my case, where I was overpaid income in 2018, then repaid it in 2019, and received a new corrected W2 in 2019, IRS pub 525 for 2018 returns states under the section called Repayments that the overpaid income must be reported in 2018 taxes as that uncorrected W2 stated, and the difference in the W2C is reported with 2019 taxes when the corrected W2 was received, and one of 2 methods is used to correct the difference
