What it costs you in storage
Ten frames of one moment cost ten photos. That is the whole arithmetic, and it is why a library can be enormous without holding a single thing you would call clutter. The photos are all real photos — they are just the same photo, repeated, and you were only ever going to look at one of them.
In a large library this is usually the biggest recoverable category there is, well ahead of screenshots and behind only a handful of very long videos.
Why iOS doesn’t catch them
Apple’s Duplicates album matches items it can identify as copies of one another — the same image saved twice, an import that ran twice, the same shot in two sizes or formats. Two frames taken a second apart are not copies: different pixels, different timestamp, different file. There is also no way to sort your library by similarity, and no “find similar” button anywhere in Photos.
What you can do about it in iOS
- Bursts are the one kind iOS does gather: Photos › Albums › Media Types › Bursts, then open each stack and keep the frames you want.
- Everything else has to be done in the timeline. Near-duplicates are almost always adjacent, so scrolling Photos › Library › All Photos by day and clearing the runs as you find them is the method that works — slowly.
- Before you start, mark the shots you know you love as Favorites, so you can be decisive about the rest.
The judgement this actually needs
“Which of these ten is the keeper” is not a storage question, it is a taste question — the sharpest frame is not always the one where everybody’s eyes are open. Kept groups the runs on the device and puts its pick in front of you, but the keep-or-clear call stays yours on every card, and every delete is recoverable for 30 days.