2 instances of Xcode installed?
If you have the latest Xcode (4.3.2 at time of writing) it should be in /Applications/Xcode. You may have an older Xcode in /Developer. /Developer is now no longer used by Xcode. You should remove it using the provided script if you don't need the old version any more. Execute 

sudo /Xcode4/Library/uninstall-devtools --mode=all


and authenticate. When that script completes you can remove the /Developer folder entirely but you should allow Xcode's script to take care of its somewhat "fragile" installation first.

New Xcode lives in /Applications and is much more like a conventional Mac OS X app these days. Lots more information about putting it in the dock, finding other developer apps, command line tools etc. in this other answer of mine.