UPDATE, June 2011: With the release of iCloud, you can now redownload music, app, and ebook purchases. In order to use these features, you'll need iTunes 10.3 or higher on the desktop and iOS 4.3 or higher on iOS devices. Learn how to redownload music with iCloud here.
Unfortunately, the iTunes Store doesn't allow you to redownload iTunes purchases that are lost or accidentally deleted. The Store is set up to require you to buy them a second time.
This applies to music, movies, TV shows, and books. I suspect this policy has to do with licensing and paying royalties to entertainment companies, but I'm not sure.
Before you go buying the item again, try reporting the issue to iTunes Store support and explaining the situation. It's possible that you’ll find a sympathetic employee who will give you a free download to replace the item you already bought. It's been known to happen (though probably won't if you make a habit of this kind of thing).
Don't expect it, though. If someone does that, it's more of a favor than a policy.
If that doesn't work, the hard answer is, you won't be able to the redownload iTunes purchase. Just like a stolen CD or broken DVD, you'll need to buy it again.
The One Exception: Apps
There's one kind of content available at iTunes that can be redownloaded: apps. Learn how to redownload apps.
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