This "problem" has been going on for a while. On one of my machines (Windows XP Professional, 32-bit), there will be the indication that updates are waiting to be applied. Looking, one update is shown, one that has already been installed. I'll say go ahead and install it, and the process will appear to work, and the event viewer states that the update has been successfully installed.
The next time the machine checks updates, bingo, same update needs to be installed. Cool, huh? Not really.
After the next general monthly update, the update that has been sticking is gone, replaced by another one.
I am using WSUS installed on one of our servers (2003 Server, 32-bit). No other machine is exhibiting this behavior.
Software/Hardware used: WSUS, Windows XP Pro, Windows Server 2003