When you save a tax file in Windows using the desktop software, it is saved only by the name that you give it (although TurboTax will suggest a name). Something like the SSN which is unique is not used in the filename for security purposes.
Therefore, if you have a tax return under the name of "Smith J.tax2018", and you create a different taxfile and save it with the same name, Windows has no way to know that you are overwriting someone else's data.
You will have to put procedures in place to ensure that you never give two tax returns the same name, or (what I would do), put each client in a unique folder so that even if you save a taxfile with the same name, it will be kept separate by using a different folder.
