This can occur on various circustances. One typical action is importing users into Exchange from various sources. One cannot have duplicate alias names for objects in an Exchange directory. See ME155414 on how to do a bulk import/export.
Build a great reporting interface using Splunk, one of the leaders in the Security Information and Event Management (SIEM) field, linking the collected Windows events to www.eventid.net.
Obtain enhanced visibility into Cisco ASA firewall logs using the free Firegen for Cisco ASA Splunk App. Take advantage of dashboards built to optimize the threat analysis process.