If my parents already filed a return and claimed me (a 20yo student), will my Turbo Tax return be able to access their return and be able to see that I was claimed?

Yes, if your parents have filed their return and they claimed you, your return will be rejected.  Your parents would have to file an amended return, and then you can file a return claiming yourself.

If your parents filed their return on TurboTax and need to add additional information, then you will have to prepare an amended return in TurboTax.  I suggest you wait at least 3 weeks before sending it to ensure that the IRS does not confuse the returns.  You have three years to file an amended return.

If you used TurboTax Online, simply log in to your account and select “Amend a return that was filed and accepted.” 

If you used our CD/download product, sign back into your return and select “Amend a filed return.” You must file a separate Form 1040X for each tax return you are amending.

Amend ONLINE    OR     Amend CD/Download       Amend 2016 - 2019

You will have to mail the amended return.  The IRS limits the system to only one Social Security number or Tax ID  to be E-Filed.  This is to guard against fraudulent tax returns.