I am having the same issue. Let's assume that on the original 1040 form AGI is 100,000. When I amend the return on form 1040X, the original amount shows as 99,000 (column A), column B shows a 1,000 and the "corrected" amount on column C is 100,000. So turbotax starts the amended return calculating that I have to pay tax on 1000, tax that I already paid with the original return. Because the corrected amount is exactly what it is on the original 1040 form. This seems to be a bug that needs to be fixed ASAP.
