craftsman tools?

i had a question and sears was already closed when i called.. but say i have a basic 151 piece tool kit, and i want to add to it so i have more size wrenches and sockets.. do i have to buy the next sizes individually or as a kit where ill end up having doubles of certain sizes? or.. does craftsman have something where it helps you build onto your tools without having to spend money on tools you already have.. i just know normally a small kit of sockets is cheaper than buying each individually.. thats why im asking.. maybe someone knows..