What it catches
True copies. The same image saved to your library twice, an import that ran twice, a photo you were sent that you already had, the same shot held in two sizes or two formats. Merging one of those sets is the safest cleanup in iOS: you end up with the best version of a photo you already owned, and the copies go to Recently Deleted for 30 days.
It is free, it is built in, and it should be the first thing anyone runs.
What it misses
Everything that is similar rather than identical. Ten frames of a burst, three tries at the same group shot, the same view a second apart — these are near-duplicates, not copies, and the album will not collect them no matter how long you wait. In most large libraries that missed category is far bigger than the one Apple catches.
Why you might not have the album at all
Two reasons, both normal. It needs iOS 16 or later. And it only appears once Photos has finished analysing your library, which it does while the phone is locked and charging — on a large library that can take several nights. An album that is missing, or that shows far fewer sets than you expect, usually just means the indexing has not finished.
What you can do about it in iOS
Open Photos › Albums › Utilities › Duplicates, then either tap Merge on each set, or tap Select and merge in bulk. Check the sets before merging in bulk — an intentional pair, like an original and an edit you wanted to keep separately, can be sitting in there.
Kept is built for the half Apple leaves behind: it groups the near-identical runs on the device and suggests one keeper, free for the first 100 photos a day.