Do I need to enter my HSA as an additional income source if it is already included in box 12 of my W2?

No, everything from the W-2 will carry over to the proper form. 

Any HSA contribution made by you or your employer are not considered taxable income. Only if they are withdrawn and not used for medical purposes will the funds be taxed. That is the advantage to having an HSA. 

The W-2 only reports contributions.  If you have withdrawals, you should have a form 1099-SSA and that needs to be separately entered as income.  It will be taxable and subject to a penalty unless you certify that the money was only used for qualifying medical expenses.  If you leave the income out, the IRS will assume you did not use it for medical expenses and will send you a tax assessment with interest and penalties.