These things can happen after updating to iCal 1.5.1:
This happens because your "play sound" alarms are converted into "message with sound" alarms.
iCal 1.5 or later allows you to attach different kinds of alarms to a single event. Earlier versions of iCal used separate alarms for "display message" and "play sound". iCal converts this type of alarm to a "message with sound" alarm, so that a persistent message is displayed at the time of the alarm in case you are away from the computer. If you already had one message attached to the alarm, you will see two after upgrading.
You can adjust the alarm or remove extra messages by following these steps: