After you file

Instead of starting over, you could just delete the duplicate W2.

Here's the general procedure for viewing the forms list and deleting unwanted forms, schedules, and worksheets in TurboTax Online: 

1. Open or continue your return in TurboTax. 
2. In the left menu, select Tax Tools and then Tools. 
3. In the pop-up window Tool Center, choose Delete a form. 
4. Select Delete next to the form/schedule/worksheet in the list and follow the instructions. 

If you want to start over, As long as you haven't submitted payment, deducted the TurboTax fee from your refund, or registered, you can erase your return and start from scratch in TurboTax Online. 

1. Sign in and select a topic to continue your return. 
2. On the next screen, from the left-side menu, select Tax Tools, and then Clear & Start Over (select the menu icon in the upper-left corner if you don't see this menu). 
3. Answer Yes in the pop-up to confirm.