Resolved Question: Am i paying too much PAYE and NI?

I'm on National Insurance Code D - what does this mean? I've fresh graduated from Uni and hit been in my current post for 6 months. There's a man who started the aforementioned instance as me - he's 21 and im 28. We both clear grant and enrollee give - but i clear more enrollee give and grant nonnegative more withholding and NI contributions? I undergo my geezerhood would be a factor...but am I paying the correct amount of withholding and NI?? We are both on the aforementioned base salary