In addition, make sure "Triggered Events" are enabled in your logging options. Then examine the domain and server logs to see if the event is showing up as having fired twice. Part of the logging information is the Event Name:
[06] Tue 24Jun14 15:33:00 - Event: USER_LOGIN (Event 03); Type: BALLOON TIP; "User has logged in"
In this particular example, the event name is "Event 03". This may help you to locate the phantom event as well.