There appears to be a bug on Apple's iTunes App Store that's causing apps to show up as having available updates even after they've been downloaded over and over again. Here's what it looks like -- the App Store on your iPhone, iPod touch, or iTunes on your PC tells you you have an update for an app. You tap or click to update, enter your iTunes password, and the app downloads -- but it still shows the app in the update list. You tap or click update again, it downloads again, but again still shows the app in the update list. Or worse (as happened to me a few weeks ago), iTunes pops up a Checking for Available Downloads dialog then proceeds to try, over and over again, for days, to download 1.3GB of turn-by-turn navigation app update.
Of course, the problem seems intermittent and random -- different users experiencing the it with different apps at different times. It's also unclear if everyone is having the same problem. For some, Apple's iTunes app servers may not be properly providing the updated app file and so the new version is not successfully getting installed on the device. For others, the file might be getting installed but iTunes doesn't recognize or register it so keeps offering the same update. We've gotten quiet a few readers writing in about it now, and there's the usual huge thread up on Apple's discussion boards.
Possible solutions include rebooting your iPhone or restarting iTunes on your PC, or just waiting and trying again in a day or so. On rare occasions it looks like it takes a few days to sort itself out, which for small apps isn't a show-stopper for those 1GB apps (or large games), it can be untenable, especially for people in countries with tight data caps on their home internet.
Some developers are hearing enough feedback on this that they're contacting Apple in hopes of some server-side fixing. We're hoping for some as well.
If you're having this problem, let us know in the comments, and let us know what (if anything) is fixing it for you.