Scenario: There's only one instance of the event in my calendar, but 15 minutes before the scheduled time Entourage pops up a notification that lists the event twice.
Others seem to have been able to resolve the issue by rebuilding and repairing the Entourage database, but that didn't work for me. Since the events I'm having trouble with are recurring events that were scheduled several months ago, I had to take a different approach.
I waited for Entourage to pop up a notification window with duplicate events. I double-clicked on one of the events to open it. When prompted, I selected "all occurrences". After the event was displayed, I appended some recognizable text to the end of the event title (e.g., "2") and saved it (ignoring all of the warnings). This allowed me to distinguish between the visible and phantom events by simply looking at my calendar. Return to the notification window, double-click the event that's not appearing in my calendar, delete it and the duplicates are gone.