(Mavericks) Appstore repeatedly updating Xcode to 5.1 (fixed?)
I was having the same problem with the Xcode 6.0.1 update continually showing up in the App Store. I applied the same update several times during my troubleshooting, even though during the process I kept deleting Xcode from my hard drive, I have no need for it any more. I didn't have more than one copy of Xcode on my drive, one possible reason for this issue.

I just solved my problem. I had disabled Spotlight indexing a while back, on the advice of a web page I had seen, because it was using too much CPU and I didn't believe I really needed it. What I didn't know was that the App Store depends on the Spotlight indexes to determine whether you have an app installed; it was looking at a stale index.

I just re-enabled indexing ("sudo mdutil -i on /"), and as soon as Spotlight actually started re-indexing, the App Store decided I didn't have any updates to be applied for Xcode.