How to remove duplicate photos in my Elements 15 catalog

I hate giving bad news, but the truth is that you won't find any 'miracle' automatic solution.

To start with, let's consider the two main problems with getting rid of duplicates.

1 - You can find dedicated softwares, some are smart, but none are 'synced' with the organizer and your catalogs. If you are happy with the way they delete (or move, rename) duplicates, that will create a number of missing files. You'll have to delete them 'from catalog' afterwards, which may take time

2 - Duplicates can be exact duplicates, which the Organizer recognizes (same date taken and same physical size in kilobytes) and refuses to import - they also can be different versions (size, format, edits...) imported at different times. You can have many such duplicates in your computer without having them in your catalog. If that is the case and you only trust your catalog, simply do a full backup and restore elsewhere; then delete all the media files on your computer. Of course, the organizer can only manage files already in the catalog. You can also have many 'visually duplicate' files in your catalog. You can manage them manually to a certain extent, but that will require time and logical processes.

My advice: the biggest trap would be to spend a lot of time and efforts without finding WHY and HOW the duplicates have been brought into your computer and/or catalogs. Once you have found out  and you decide for a better controlled folder location for your downloads, we can try several tools to help you manage your duplicates problem. The solution given by Arshla is one of them.

You can take advantage of date_taken, keywords, captions to find batches of duplicate imports. That will be more efficient than dealing with single duplicates...

Since the problem arises more frequently on scanned or not camera imported items, I have also chosen to give up deleting duplicates I have found. If I can stack them, that makes my searches in the organizer more efficient. I don't save disk space, but the searches, whether by keywords or visually are perfectly working.