Hi,
When using the Photos app, any content placed in albums would not be duplicated. It will be the same photo file in the preferred album; regardless of how many albums contain that photo. If you delete the photo in one album, it will delete it from all, and from your photos. This applies to iCloud Photos and the Photos app.
Photos deleted are then moved to a ‘Recently Deleted’ album, and have the option to be recovered.
For duplicates, there is not a way to see what duplicates you may have. I would recommend downloading a third-party app on the App Store that offers the feature of sorting through your duplicates.
Another alternative would be to create an album, title it ‘Duplicates’, and then manually go through your photos and add all the duplicates you see to that album. Once you are done sorting through your photos, go to the album and delete all the duplicates except one.
Separating photos into new albums does not double the amount of storage space required, as you are never duplicating the photos being added to the preferred albums. It will always be the same photo file.
Hope this helps.
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