We know that the IRS is already revising the 2020 Form 1040 to include the 2nd stimulus, but that change will entail a lot of re-programing at the IRS and for the tax software programs. It may be at least a month — or longer— before the forms are changed. Do NOT be in a hurry to file your 2020 tax return until more is known. Give some extra time to the IRS and the software programmers so that the software can handle the new 2nd stimulus, so that you can get it all by filing ONE time. It is never a good idea to file too early; filing your 2020 return too soon may result in a lot of extra confusion that can be avoided if you wait and file a little later.
