The Mac version of Duplicate Photo Cleaner has a special scan mode to find duplicates in the Photos library. This mode is very efficient and is absolutely safe to use because deleted duplicates are first moved to the DPC Trash album within Photos, so that you can examine the results in detail.

To scan your Photos library for duplicate images, select Photos Scan as the Scan Mode using the drop-down menu in the top-right corner. You will be asked to select your Photos library and then the albums where you’d like to look for duplicates and similars:

When you're done selecting folders, press OK and then click on the Start Scan button.

When you’re done selecting folders, press OK and then click on the Start Scan button. Wait for the scan to finish (scan time depends on the amount of photos you’re scanning for duplicate and similar content). Then 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:

Select all the images you want to delete permanently, press Command + Delete and the duplicates will be gone.

The duplicates will be placed in the DPC Trash album in Photos.

Now open Photos and open the DPC Trash album. Select all the images you want to delete permanently, press Command + Delete and the duplicates will be gone.