My employer already withheld the additional Medicare tax. and Turbo tax is calculating and adding the additional medicare tax again, can you please help?

[dmertz]

Your actual additional Medicare tax is determined on Form 8959 line 18 and propagates to Schedule 2 line 11.

The additional Medicare tax withheld by your employer is determined on Form 8959 line 24 and is credited back on Form 1040 line 25c.