TurboTax may have a few screens which ask you to verify your wages after finishing your W-2 entry, in which case it is okay to enter this information again. But, you should only complete one single entry for each W-2 you have received within the Wages and Income section of TurboTax.
If you have multiple entries for the same W-2, this may cause your income and other information that is reported to be doubled (for example, your reported income may instead be reported as $60,000 instead of $30,000).
You can verify the number of entries within the program by clicking on Wages and Income - Wages and Salaries, and then clicking through the screens until you see Here's Your W-2 Info. Any duplicate entries on this screen should be removed.
This just happened to me, here's what I did: go back to the screen where you can view your documents. For me, I saw two W2's. One showed $0.00 and the other showed my income. Once you start the filing process, I think TurboTax clears your 2019 W2 and replaces it with your 2020 W2. I located the W2 showing 0.00, looked over to the right and clicked the trash can icon to delete it. Once this was done, I was able to move through the Review step. My income was correct (not doubled) and my tax return looked correct as well. Hope this helps!