To create test account on TT Online, can I duplicate existing account without reentering all info again?

[VolvoGirl]

No.  You can used the Desktop program to save a copy to test in.  

[VolvoGirl]

So do not use your real account/return to test in.  You can use the same email for 5 accounts.  You don't have to pay for an account unless you want to print it or file from it.

How to start another return in the Online version

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

The Desktop program can do unlimited returns and has other features and advantages over online.  You can copy your return and it has a What-If worksheet you can use.