So as you said you received the taxable SSD check at the end of 2017 and paid back in 2018; actually as a cash method user, you had to report the SSD income on your 2017 return however as you said you paid it back in 2018.So in this case, since you already filed your 2017 federal tax return and then discover a mistake , you need to fix errors by filing 2017 1040X; you had to report your 2017 return to report the SSD income on your amended 2017 return and of course (most TPs use cash method),since you owe additional taxes with Form 1040X for 2017, you need to file it and pay the tax as soon as possible to minimize interest and penalties.
Also since you paid it back in 2018 you also need to file 2018 1040X for refund. For your refund, Generally, you must file Form 1040X for 2018 return within three years from the date you filed your original tax return or within two years of the date you paid the tax, whichever is later |