Code DD is the non-taxable value of your HDHP, *not* your HSA contributions.
Employer and pre-tax employee contributions to your HSA (the most common types) should be reported as a single total in Box 12 with code W; they should *NOT* be added to code DD.
If your HSA contributions were made after-tax (either by payroll deduction or directly), they should not appear on your W-2. Those should appear on a Form 5498-SA from your HSA provider.
