Deduplication is used by the official Flickr Uploadr app (at least on MacOS) which is what actually brought me here. The app only adds the photos that are not duplicate to the album resulting from a folder upload. Having to add the duplicates to the album by hand sucks which is why I decided to write my own uploader - do not be mistaken, I like the deduplication feature, it's just the official app is very inflexible.
My guess is stat value '1' means "hard" dedup. When used, it actually detects a duplicate of a photo that was deleted, however it will not add it - the 'duplicate_photo_status' XML element in the response says 'DELETED'. The value '2' seems to be a "soft" dedup - the 'duplicate_photo_status' XML element in the response says 'OK'. The photo is uploaded, or rather the deleted photo is resurrected - I still need to check this.