I had the same issue , could not enter Calif State Refund for 2016 which I received in 2017, TT feedback on red text "you already have entered data for this State/Locality". I was able to enter my CA State Refund as follows: For State, I entered Arizona instead of CA, then I was able to enter the State Refund, I then went to Forms and corrected the State info on the Federal Carryover Worksheet.
I spent 1 hour on the phone with TT support, they reommended deleting the Federal Carryover Worksheet but I did not want to re-enter all the data again. It is a bug that TT needs to fix