Why does Xcode still show up as an available update in App Store after being installed?
Was having the same problem after installing Xcode v4.6 on a system with Mountain Lion, had Xcode v4.5.2 &amp; v4.4.1 (with different names) also installed in the Applications directory. Read on a different thread that Spotlight indices can sometimes cause problems, followed the steps outlined here:

http://support.apple.com/kb/ht2409

This fixed my problem. (YMMV)