identify and delete duplicate photos

Photos is checking for duplicates on several occasions, but it does not give us an opportunity to use the results to remove duplicates.

• Photos is checking for duplicates, when we are importing photos and alerts us, tat we are about to import duplicates.
• Photos is checking for duplicates, when we are merging libraries from several devices in iCloud, so there will not be duplicates in iCloud.
• Photos 5 is checking for duplicates, to present a clutter-free view of our photos in the Days view. Unfortunately, we cannot reveal the rejected duplicates. But we can use it as a hint, where to look for duplicates.

See: About the Duplicate Detection in Photos 5 - Apple Community

Im preparing to back up and move my photos (and everything else) to a new MacBook (from my old one). I have around 60000 photos, and there are many duplicates - tho I don't know why. Should I utilize PhotoSweeper before or after I transfer my data to a new computer? Any advice?

If your new Mac has enough free storage, do the cleaning after the transfer to the new Mac. Then you will have the original library as a backup, if you make a mistake when deleting duplicates.

Even with an automatic duplicate removal you should check all marked duplicates before actually deleting them. You want to be sure, that the photo with the better resolution will be kept, or the photo with more metadata, the photo added to an album. You can configure Photo Sweeper to pick automatically the photo with more pixel or more metadata, but check the photos suggested for deletion. I am only using PowerPhotos or Photo Sweeper to suggest candidates for deletion, and do not allow to delete the duplicates sight unseen.

No. You'll need to use a 3rd party app. You want an app that will identify the potential duplicates, put them in an album or mark them with a keyword for easy retrieval and deletion by you. You don't want one that does the deletion itself for obvious reasons. 

I've run tests on the following apps with these results:

PhotoSweeper - $9.99 - Demo version available. 

PowerPhotos - $29.95  

PowerPhotos is the iPhoto Library Manager version for Photos and is very powerful. Although more expensive I would recommend it as it has more capabilities than the others like the capability to merge Photos libraries or copy photos, both original and edited, along with their metadata between libraries.

PhotoSweeper compares bitmaps and/or histograms so it can detect duplicate images even if they have different file sizes, file names, image sizes and capture dates.

I see here that there are 3rd-party apps which purport to do what you want. I wrote an AppleScript which adds duplicates (if they're side-by-side) from my "Everything" album to my "redundant" album. Then I can delete the contents of the "redundants" album, after manually checking for sanity.

It checks about one photo per two seconds, which is slower than just deleting them by eye. I hope the professional apps are faster.