C., I would say that it's not appropriate to have a second shower at this time. The things that make it ok are these:
1. You're having a baby of a different gender than the first
2. You're having another baby after several years and hadn't planned for it so gave all of your old stuff away
3. Something happened in your life that has left you without your first batch of baby gear (i.e. house fire, natural disaster etc.)
I would say no to a shower this time around, just tell her that you don't feel right about it because you already have so much stuff. Tell her that you will register in case anyone is dying to get something for the new baby, but you'd just as soon re-use what you already have.
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Sorry to edit my response, but I came back and read some of the other responses and I feel compelled to add a few things.
Let's talk about definitions here: The definition of a baby shower is a party given FOR the new parents (not FOR the baby) for the express purpose of helping offset the financial burden of having a baby. A shower invitation, by definition, brings with it the REQUIREMENT to bring a gift. The purpose of a shower is to provide the new parents with a great deal of the items they will need for the baby.
Parents that already have a baby, already have a great many of these items and it would be greedy to have a party where people are REQUIRED to bring gifts that AREN'T necessary.
I am not saying that each life shouldn't be celebrated, by all means, have a welcome home baby, or happy impending delivery party if you want in order to celebrate, but a shower is meant to "shower" you with gifts. Gifts which you don't need and in this difficult economic time, would be rude to ask people to spend money on.