how to remove duplicate photos

Make a current backup of your Photos Library, before you start deleting photos.

The Photos.app on the Mac does not have an inbuilt tool to help us to search for duplicates. It is checking for duplicates, when we import photos and warning us, when we are trying to import the same file twice, so there should not be any exact duplicates. If we import the same photo in different versions (not as identical files) we have to find them on our own.

You should be seeing the duplicates, side-by-side, if you browse your photos in a view, that is sorted by the capture date, for example in the "Moments".

You can also use a third-party application to scan for duplicate photos, but be sure to use a dedicated duplicate scanner for photo libraries. General cleaning tools, that do not use the Photos.app to access the Photos Library, can damage the Photos Library and cause data loss. Apple is warning about cleaning applications, because they have caused many photos to be lost: Using third-party apps to remove duplicate photos might damage your Photos for macOS library - Apple Support

Power Photos is safe to use, as far as I can tell, also Photo Sweeper.

To add to léonie's comments: 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.