Enter amounts shown in box 12 of your W-2 only in box 12 of TurboTax's W-2 form, nowhere else in TurboTax. The items in box 12 pertaining to your 403(b) contributions will have code E or code BB. the amount reported with code E has already been excluded by your employer from the amount in box 1 of your W-2 so you don't get to deduct that again.
