For some strange reason Apple iTunes refuses to update correctly. It always helpfully offers to download and update iTunes (and usually QuickTime as well). But it always fails for some reason.
The iTunes updater has a “download only” option. When I tried that today and then tried to run the update separately I received this error message:
Google searches for the “A later version of Apple Mobile Device Support is already installed” weren’t very helpful. None of the results addressed the problem (or even said what it really was) or provided any viable solutions.
I ended up having download the whole iTunes install package from Apple.