Photos

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 these two apps with the following results and found them to be safe to use:

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 versions, along with their metadata and albums between libraries.

PhotoSweeper - $9.99 - Demo version available.

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.

As a start, you could use a smart folder to get all the pictures within a certain date range, sort that album by oldest to newest, and hope that duplicates will appear next to each other. This will work as long as the dates that Photos sees have remained undisturbed. There are other choices in the smart folder settings that may help-- you can select only photos taken with the same camera, for instance. But if any of this metadata has been changed, it will be much harder.

The apps that Old Toad suggested don't actually delete anything-- they just move pictures to a separate folder that you can look at later, so actually deleting them is another step, Finding and deleting duplicates is can take quite a bit of time, even if you use helpful apps. You may reach a point of diminishing returns and decide it's not worth it to go further.

You could do this: Use a smart album, set it to Album-IsNot-Any, and you'll get all the photos that are not in any albums. You need to mark these so you'll know them when you're choosing which ones to delete. You could Select-All and make them all favorites, so you'll see a little heart on all the pictures that aren't in albums.

But you may be using the little heart already and you don't want to mess that up. If so, use Keyword to mark the ones not in albums. Once you have all the pictures that are not in albums in the smart folder, select them all, open the Info window, and enter NoAlbum (or some such) as a keyword. Now, if you keep the info window open as you look through all the pictures, when you see two identical photos, clicking on one of them will tell you if it's the one in an album. The only difference with this method is the marker isn't on the photo itself, but rather it's in the info window, so you have to look back and forth.

Good luck...

Yes it will. You also want to find out why you are getting duplicates so that you don't have to keep doing it in the future.