If you are using the Online version, you will have to do a new, separate online account (with a different user ID) for each federal return you want to prepare and purchase for each (if you need to use a paid version of the online software). If you do another return under the same username you did your own return, it will override the one you already filed and lose everything for that return. It's always been this way online.
· Start another return in TurboTax Online: https://ttlc.intuit.com/questions/1894512
You can still use your same email address (for up to 5 online accounts) if you want to get notifications, but the username must be a new one for each online account you create.