Hello.
As always, it is strongly recommended that you have a current Time Machine or other backup of your data.
-Which version of OS X/macOS?
-Which version of iPhoto?
The best way to remove duplicates in iPhoto/Photos, is manually in the application itself. It can be a daunting task with a large library containg hundreds/thousands of photos. There are several applications that remove duplicates, but I personally haven't found them to be very successful.
C