That information is denoted in the wages section and is pulled from box 12, letter codes D and AA. However, when you move to the credits and deductions section, is again asks if you have made contributions to your 401k. I have but if I enter this detail, am I double counting? this isn't very clear.
You only input IRA contributions under Deductions and Credits. The retirement plan contributions on your W2 are not IRA contributions. So, entering under IRA contributions would not be double -counting... it would simply be incorrect.
If you contributed to an IRA, you would not have done so through your employer (except in rare cases). Your W2 may have a 401(k), 403(b), 414(h) or similar contribution. These are accounted for simply by typing in your W2.