The Mac version of Duplicate Photo Cleaner has a special scan mode to find duplicates in the iPhoto library. This mode is very efficient and is absolutely safe to use because deleted duplicates are first moved to iPhoto trash.

To scan your iPhoto library for duplicate images, select iPhoto Scan as the Scan Mode using the drop-down menu in the top-right corner. iPhoto will open automatically and you will be asked to select the folders you want to scan:

When you're done selecting folders, press OK and then click on the Start Scan button. Wait for the scan to finish.

When you’re done selecting folders, press OK and then click on the Start Scan button. Wait for the scan to finish.

Click on the Go Fix Them button to examine the scan results. Select the duplicates you want to delete and use the icons in the top-right corner of the program window to manage the duplicates:

Click on the Go Fix Them button to examine the scan results