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

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.