Double click prevention introduced in Drupal 7

By default Drupal will now prevent consecutive form submissions of identical form values via a new separate library drupal.form-single-submit. Repetitive form submissions that would submit the identical form values are prevented, unless the form values are different from the previously submitted values.

You can opt out of this behaviour by setting a variable as described in default.settings.php:

/**
 * Opt out of double submit protection.
 *
 * By default Drupal will prevent consecutive form submissions of identical form
 * values. Set this variable to FALSE in order to opt out of this
 * prevention and revert to the original behaviour.
 */
# $conf['javascript_use_double_submit_protection'] = FALSE;