Announcement: There is no good server-side solution for this problem in Drupal 5 or Drupal 6. Your best option is Ted Serbinski's JS solution: http://tedserbinski.com/2007/01/11/how_to_prevent_duplicate_posts
That has been turned into a solution at http://drupal.org/project/hide_submit
Historic page contents
The Formsingle module prevents forms in Drupal 4.7 from being submitted twice. Try this; with your Drupal 4.7 installation, create a new page, or submit a new comment. When it comes time to press the submit button, click it as many times as you can before the page reloads. Can you click it 50 times? 100 times? Does Drupal submit a new page or comment for every single click? You bet! This module fixes that.
As this is a new module and totally untested in the real world, I highly recommend NOT installing it on live sites. Please test this module, examine the code, provide feedback, and hopefully soon we'll have a solid tool to provide this (critical) feature.
See the Roadmap issue in the issue tracker for an overview of tasks and future directions for this module.