If an item is saved 3 times, the event will be fired 3 times.
How do you set up the event handler? Is it possible that you set up the event hook 3 times?
How many times does even fire if you look at the folder in OutlookSpy? Click the Folder button on the OutlookSpy toolbar ("Outlook Object Model" group), select the Items property, click Browse. In the new Items window, go to the Events tab. Modify an appointment, look at the Log box at the bottom of the Events tab.