Everything said by the other monks is true. But you can come close to blocking one-session multi-submits by generating a random word or number in GD, and displaying it on the submittal page, then checking if the user wrote it right. See merlyn's WebTechniques column for an example of this. ("Keep robots from stuffing your forms", I think)

However, this relies on people's laziness, which is generally a good bet, but may not be applicable sometimes :) Certainly amongst Perl programmers though...