Track contacts who attended more than 1 event
You can easily GROUP BY and COUNT() in mysql - do you have access to mysql? If not you can also put this in to a CiviCRM custom report. 

Here's the query:

So below contact_id 2309 participated in 5 events; contact_id 3186 in 2 etc. 

    mysql&gt; SELECT contact_id, COUNT(*) from civicrm_participant GROUP BY contact_id HAVING COUNT(*)&gt;1
+------------+----------+
| contact_id | COUNT(*) |
+------------+----------+
|       2309 |        5 |
|       3186 |        2 |
|       3568 |        2 |
|       5820 |        2 |
|       7007 |        3 |
|       7045 |        3 |
|       7048 |        3 |
|       7083 |        3 |