How do I remove duplicate photos in a shared iCloud folder?

Have had same problem. The iCloud Photos user interface provides no guidance, no feedback, no way to review selections, no way to confirm/deny. The "Add Photos" mechanism is a joke - doesn't even let you select an unshared album to include in the shared. Once you select the photos to upload, you have no way of knowing what - if anything - is happening. The upload process is so needlessly long that it seems it has failed, so you end up reselecting the photos, resulting in duplicates.

The only way I've been able to get around it is by deleting all items in the shared album, then reloading - it seems that the reload is fast, even though the original load and delete are slow.

I loaded the same albums into Google Drive and Amazon Prime Photos. These GUIs are much better than iCloud's. You simply pick out the file(s)/folder(s) you want to upload, and it gives you file-by-file feedback - together with the amount of time remaining for the entire process, etc. Then, once the process is complete, the photos appear online in a sensible order. Unlike the Apple version, where there is a random mish-mosh, the Google and Amazon GUIs display your pictures in chronological or geographical order. I have no great love for either Google or Amazon, but in the cloud photo business, they're the run-away winners.

What exactly are you calling "shared iCloud folder"

That is not a feature of iCLoud - there is iCloud Photo Library, iCloud Photo Sharing, MyPhotoStream, iCloud Library and other iCLoud services - which one are yo enquiring about?

LN

It's a shared Album in iCloud Photo Sharing/

There is no tool to screen a shared album for duplicates. You cannot even sort the photos by name or date to see the duplicates side-by-side.

Either select the duplicate photos individually to delete them, or select all photos at once in the shared album to delete them all together and then upload only the photos you want again. To delete selected photos from a shared album select them in Photos, then ctrl-click and use the command "Delete from Shared album":