They may think they have just about everything they need.. but chances are they do not..
When I first got married I thought I would have a surplus of "stuff".. I had a fully furnished apartment.. my then fiance also had a fully surnished apartment... we thought we would have tons of duplicate "things" etc..
We were wrong.. moving from a couple of little 800 square foot 1 bedroom apartments into a 2100 square foot home meant there was a whole lot more space.. more stuff was needed to cover walls, floors, etc.. more furniture was needed.. etc..
and then we dealt with just wanting to get rid of old "bachelor" grade stuff (pots, pans, knives, furniture, etc..).. and upgrade to "family" grade stuff.... the old half broken, stained bachelor pad couch and recliner wasnt good enough anymore..
It is highly unlikely you can go wrong with anything you would get them..
The biggest problem we dealt with was duplicate "stuff" (ending up with 4 toasters, 3 crock pots, 10 sets of bath towels, etc..).. we didnt have a problem with having too much or anything "wrong"...
All that said.. I agree with most of what has been posted above.. anything personalized is a great idea.. as are things that go on the patio.. porch.. etc..
At the end of the day.. even if they already have whatever you want to give them... they own it individually.. when you give them an item as a wedding present they suddenly own the item "together".. and will likely discard the old item and replace it with your gift....