How to delete duplicate photos from an iPhoto event?
Just because there are duplicates within iPhoto, doesn't mean that iPhoto actually has duplicated the file.  Navigate to ~/Pictures/.  Right-click on iPhoto Library, show package contents.  Find your event within Originals/&lt;year&gt;/&lt;event name&gt;.  Backup the directory.  Remove the event from iPhoto - I don't know if this removes the actual folder or not, that's why I suggested backing it up first.  Re-add it.  Voila.

Let me know if it has actually duplicated the files.  I know of a great duplicate removal app, but it's not free.