Photos shared album show twice for each photo, how to remove duplicate?

The normal tool for removing duplicates doesn't apply to Shared Albums. You'll have to continue the manual process.

A bit of background information on Shared Albums.

The name "Album" for shared albums is a bit misleading. They are just streams of photos and not a part of our Photos Library. They are just shown in Photos for convenience and do not behave like albums at all. They have been introduced in OS X 10.8 Mountain Lion as a companion to My Photo Stream for iPhoto and Aperture and have initially been called "Shared Photo Streams". When Apple discontinued Aperture and iPhoto the Shared Photo Streams have been renamed to Shared Albums in Photos, but they are still streams and not standard albums.

Some differences between standard albums and Shared Albums:

• When we add a photo to a standard album, the album does not store the photo, just a reference to the photo in the library. But when we add photo to a Shared album, a second version of the photo is added to the shared album, downsized and with metadata removed, just for sharing.
• There is no connection between the photos in the shared albums and the original version in our Photos Library. When we edit a photo we just shared, the changes will not be applied to the shared photo in the Shared album. The shared version will not be updated. We have to share the photo again and remove the first version we shared.
• We cannot add the same photo to a standard album twice, but we can add the same photo to a shared album over and over again.
• Shared albums are sorted by the date the photos have been added or the date a comment has been added. We can sort the shared album on our Macs (only in the most recent versions of macOS) but this will not sort the Shared albums in iCloud or on the devices of other subscribers.