I received a corrected W-2 from my employer for box 12. Code entered was D and should have been DD. Do I need to amend this info on my return?

It depends.

You only need to amend if the correction changes your total refund or tax due.  

If you have already filed and return has been approved, please click here 2018 Amend.  Follow the instructions in the program and it will tell you if the correction changes the result.  If it does make a change, you would need to submit your amendment by mail.