I've been to several baby showers, and had a couple myself. I don't think anyone is all that concerned with the cost of the gift. The fact that you have gotten things all ready shouldn't keep you from a gift at a shower. Those are extras, and I have been in simular situations and would feel wierd not bringing something to the shower. As some ladies have said, it doesn't need to be an expensive gift, but it should be thoughtfull. Think about the kinds of things that would be very nice to have when the baby is around. Items of convenience, things to help with comfort, things to either ocupy or carry baby while mom and dad are busy, clothes that will be needed yet haven't been purchased, a night out alone for the parents, anything that gives some thought to what will help the baby feel comfortable, or give mom and dad more piece of mind. The last shower I went to I gave a jumbo pack of stage 2 diapers from a wholesale store because they will go through them and diapers can get pricey, I also got a combo pack of medicines and infant care stuff because it's nice to have whatever you need when the baby gets sick or hurt. I had already given an activiy blanket earlier that I had previously purchased. When I wanted one for my son I was not impressed with the prices on what I could find. After the fact I found this great activity blanket with all sorts of learning things, bold colors, and a good price, so I got it early since I knew they were having a baby and could use it to place the baby on when they don't want him on the dirty floor.
Just keep it thoughtful, and try not to buy a lot of things that everyone will buy. If you want to do something for the parents I would find out what kind of food they like and get a gift certificate, put it in a card with a note that you will watch the baby for a night so they can go out alone.