How to remove multiple duplicate photos

Do you have My Photo Stream enabled? It can create duplicates, if you are also importing manually.

• Another potential source of duplicates is shooting the photos on the iPhone in the High efficiency format HEIC or videos in HEVC, but importing them in a more compatible format. The HEIV files on the iPhone will always appear as new photos, because you are nit importing them but converted versions.
• Or iCloud Photos can be causing duplicates. If your iPhone is syncing with iCloud Photos and continually downloading the same files from iCloud over and over again, the new downloads will be appearing as new photos on the iPhone.
• Several users reported, that their WhatsApp download are also appearing again and again as ne files in the Import. I cannot test this, because I am not using WhatsApp.

The safest way to avoid duplicates is to stick with only one method of syncing, only My Photos Stream, only the manual download, or only iCloud Photos. But do not mix the methods. and avoid any format conversion when downloading from the iPhone. My Photo Stream seems to become deprecated by Apple and cannot transfer videos or Live Photos. I would stick with the manual import or with iCloud Photos, but not mix them.

We have to detect the duplicates ourselves and then pick the ones we want to keep.

Photos is searching for duplicates, but only for a better layout in the "Days" view. It is suppressing duplicates there for a more pleasing layout. But it does not show us the duplicates for removal.

There are a few free apps, but you have to be very careful which one to use.

The only duplicate detection apps I know that I have tested, that are safe to use (afaik) and do not destroy the library are not free (PowerPhotos or Photo Sweeper). Other free apps I tested tweaked the system and installed additional spyware to collect our data, so we are paying for the free service with a flood of SPAM or worse. Stay away from Photo Duplicate Cleaner for example or other apps by the same manufacturer. When I tried it, MalwareBytes quarantined the app because of system tweaks for data collection.

Using third-party apps to remove duplicate photos might damage your Photos for macOS library - Apple Support

+1 to léonie's recommendation of these two apps: 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 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.