Looks to me that you are still overpaying, by paying the additional medicare tax twice - once as shown in your W2 and the second time in the Turbo Tax calculation. As you stated, the total medicare + additional medicare tax should be $4005. Your W2 shows you have already paid $4550. Turbotax is ensuring that you receive the $450 refund from your W2 overpayment of $4550. But you've stated that Turbotax is also adding the additional medicare tax of $145.89 to your TurboTax calculation of taxes due, when this amount was paid in your W2 (ie, $3860.05 + $145.89).
I'm having a similar issue with Turbotax adding the additional medicare tax to my overall taxes due, when this was paid accurately during the year, as shown in my W2.
