I think each baby deserves a shower...it is the baby shower so for the most part, the gifts are for the babies. I will admit that the subsequent showers should not be quite the same "event" but rather something smaller. For example, many go all out for their first so their second should be more subdued. Each baby will still need diapers and wipes, other supplies such as that. They also deserve a few pieces of new clothing especially if they are of a different sex than their older sibling. Depending on the age of the other sibling, a new car seat may be in order. The larger the age gap, the more necessary the next baby shower.
In my case, my son and daughter are 15 years apart. I had some new and some used things with my son and naturally didn't still have them. Naturally, there was a shower for my daughter and again I had some new and some used things (and lots of new clothing!).
I should probably mention that my kids have different fathers and our daughter is my hubby's first baby.