On my w2, in box 12 D is the amount of my contributions for my 401k. Do I enter that same amount in the section for IRA contributions?

No; an IRA is *not* a 401(k), and vice versa. You enter your 401(k) contributions in W-2 Box 12 with uppercase code D, just as your employer reported it. (If the uppercase code is *not* D -- whether the lowercase Box 12 line is a, b, c, or d -- your employer needs to correct your W-2.) Your employer has already deducted the contributions from W-2 Box 1; you do not deduct them again.