I want to file taxes for another person w/ existing TurboTax account but it preloaded last year's info. I tried to open a new account but email address is already used

If you're using your online account and you use your ID and Password, you will get your information pre-posted.

Simply create a new ID and password for the other person and star a new tax return.

You can use the same email for up to five accounts but each return needs its own account and user ID.

 

When you use online TurboTax software you get one return per fee.

 

Each return needs its own account and user ID.   If you use the same account and user ID for a second return, the second one overwrites the first return and it is lost forever.  

 

https://ttlc.intuit.com/community/using-turbotax/help/how-do-i-start-another-return-in-turbotax-online/00/25596