Check with the store you're registering at to see if they give discounts to you for fulfilling your list after your shower. If so, register for the big ticket items (glider and breast pump, etc.) as well, so you can get them cheaper after the fact, even if you're not expecting anyone to buy them for you. We were able to get the Medela PiS with gift cards mainly from our shower, and got it at a discount ourselves.
Do NOT register for two different types of one item, unless you're having two babies. It's confusing to the buyers. Research the best prices and safest/highest reviewed items and go with one of each bigger item (swing, car seat, stroller, etc.).
If you're concerned about exposure to off-gassing, go with wool or tightly woven cotton puddle/lap pads instead of vinyl mattress pads.
When choosing outfits, make sure embroidered items have a backing so it's not scratchy underneath. Don't forget to register for some 3-6 month (and up) clothes too. I wouldn't register for anything beyond 6 months, personally, though. Don't forget socks and hats/caps.
You cannot have too many receiving blankets. Seriously. These things work for all kinds of messes and scenarios beyond swaddling.
Splurge on a papasan swing if you can. We have one and it was a lifesaver (or get an Amby Bed!).
Keep in mind that essentials for some babies are total wastes for others, though. So save your receipts and the original boxes!!! Even if you end up using something a lot, having the original box can bring you a little more money if you resell it later on down the road.
The more bells and whistles something has, the more things can break or be pulled off.
The more nooks and crannies a high chair has, the bigger pain it is to clean.
Get rechargeable batteries. And a charger. Register for those too.
If you and/or your partner were smallish at birth, you may want to go with an infant bucket carseat instead of a convertible, initially. I was a small baby (6#5oz) and my daughter was too (6#6oz) and I'm very thankful we had an infant carseat. The convertible seat would have swallowed her. We went with a travel system because the two items together (seat and stroller) were cheaper than buying them separately.
We registered at three places: BRU, Target, and www.nurturedfamily.com
We researched everything and chose these three places because they had what we needed, with the *best* prices. We registered for our diapers at NF.com, and then between BRU and Target registered for what we needed, at whichever store had the best price. We did not register for the same items at both stores. We had no need to double up. So figure out what you need, which items are the best quality/reviews, then find the places that are convenient and/or affordable to register for those items at.
I don't understand why some mamas go nuts registering for things they don't need. We only registered for what we needed and we actually had several people comment at the shower how easy it was to find something off the registry. So from my anecdotal experience, I vote don't go nuts.
Splurge some, sure, but don't overwhelm people or yourself.
Research first, then find the best prices, then register.
Good luck mama and have fun!