This is another reason we recommend being very lax with your download attempt/expiration settings, and generous with reactivating or resending download links when you can verify that customer has already paid. Any number of technical factors can interfere with the download process, or glitch up what would seem to be a successfully-completed download but result in an unopenable file.
Giving your paying customers every possible chance to accomplish a successful download, and giving them the benefit of the doubt when they claim their downloads have been unsuccessful, will tend to minimize the likelihood of complaints and chargebacks. Anything less will not deter piracy or "double dipping" one bit, and will only tend to confuse and annoy your honest, paying customers who may simply have a busy schedule or a lame computer or ISP and need to keep trying several times, or try at different times of day, just to get one good download.
Anyone who's inclined to be a dishonest crook does not need to re-download your file more than once if they want to be a pirate -- they can just copy or share the file they already downloaded the first time. Indeed, the copyright-law doctrine of "fair use" supports consumers' right to make personal-use copies of any work we've already bought a copy of -- this is why you can buy a CD and rip it to mp3 files for your personal use (e.g., to add to your computer's jukebox playlist, or put on your iPod), but you cannot legally share those mp3s with anyone else. Aside from somehow building into the file itself some kind of restrictions on copying or access (such as SimpliFun/Lou Anne mentioned above), it is simply the nature of this digital world that any file, once it's been downloaded to any computer, can be copied on that computer and shared with other computers.
You just have to trust that the few dishonest customers who might copy or share your stuff will be far outnumbered by the vast majority of your honest customers who will observe the "honor system", pay for what they receive, and not share what they bought with others. The more generous and forgiving you are with your honest customers, the better a reputation you will build, and the more honest customers you will attract.