I created a content type with a cck date field and enabled the functionality of repetition. When I created a node of this type, I put granularity as "day" and told to repeat the event for the next 5 days. When the cron ran, I received 5 emails warning. To fix this, I made a small change in file includes/cron.inc close to the line 57.

I've just modified it:

<?php
while ($signup = db_fetch_object($signups)) {
  $user_mail = _signup_get_email($signup);
  $params = array(
    'subject' => $subject,
    'body' => $node->reminder_email,
    'node' => $node,
    'signup' => $signup,
  );
?>

for this:

<?php
while ($signup = db_fetch_object($signups)) {
  $user_mail = _signup_get_email($signup);
  // send only one email per user
  if (in_array($user_mail, $nodes[$node->nid])) {
      continue;
  }
  $nodes[$node->nid][] = $user_mail;
  $params = array(
    'subject' => $subject,
    'body' => $node->reminder_email,
    'node' => $node,
    'signup' => $signup,
  );
?>

Hope that helps on something.