If I already filled out box 12D on the wages and income section for my W2, do i need to fill it out again in the deduction and credit section for retirement funds?

NO!  Only enter it from the W2.  That is 401K not an IRA.  Only IRA contributions go in Deductions & Credits.

Thanks for clarifying! Super helpful! Also, another question, is there a (max) limit on 401K contributions like IRA contributions?

Yes 18,000 or 24,500 if over 50.  Check with your employer.