My W2 box 12 has D 20 23.65 D 4910.74 What is this and what amount do I enter

Enter both of them.  The "20" on the first line refers to the year 2020.  Ignore the 20.  Just enter code D and the amount.  Then enter another row for the second code D and amount.

After I enter both I get error message saying I have duplicate entries 

You can add the amounts together and make one code D for the total.

Where does it say I can combine them?