If my employer is deducting money from my check for a 403b. Can the money deducted be used as a deduction?

You will be entering that when you enter the information from your W-2.  Do not try to enter it again anywhere else.

No.  It has already been deducted from the W2 wages in box 1 and you can't deduct it again.   Notice that box 1 should be less than boxes 3 & 5?