Unpack in your modules folder (usually /sites/all/modules/) and enable under Administer > Site Building > Modules.
Here are some key features of "Block submitting twice":
· Disable all submit buttons of the form currently being submitted. It can add a css class to them, append a text to each of them or add a text next to the buttons.
· Hide all submit buttons and replace them with a div, whose css class and text contents are configurable.
· Drupal 6.x