Was it a Roth 401(k) or a 401(k). "Regular" 401(k) contributions are already NOT TAXED, so your W2 Box1 income reflects the deduction. Roth is not pretax.
You can never deduct your 401k contributions on your tax return, as there's simply no place to do so.
401k contributions go in box 12 of your W2, with a Code D. Please look at the hard copy of your W2 to see if its there.
If your income is low enough, the code D amount could give you a small Saver's Credit on your return. If you don't have a code D, report back to your employer for a corrected W2.
401(k) contributions already lower your taxable wages, so they cannot be double-deducted on your tax return.
