Short answer: the best cleaner app for duplicate photos on iPhone is the one that makes exact repeats obvious and reviewable without mixing them into harder similar-photo choices.
That distinction matters because duplicate cleanup should feel calm and fast. If a tool blurs exact duplicates with near-duplicate keepers, it stops being a duplicate-photo cleaner and starts becoming a riskier general photo sorter.
Why exact duplicates are not the whole storage problem
iPhone can already help with exact duplicate detection, but many bloated libraries are really full of similar shots, edited copies, and repeated saves from apps. If a cleaner app only promises duplicate removal, make sure that narrow promise actually matches the kind of clutter you have.
What the best duplicate-photo cleaner should do
Separate exact duplicates from similar photos clearly.
Show grouped review so you can confirm the keeper quickly.
Make low-risk duplicate cleanup finishable in one sitting.
Help you move into broader library cleanup only after the obvious wins are done.
What privacy-minded users should look for
On-device review instead of vague cloud-processing claims for your photo library.
Clear keeper selection before anything moves toward deletion.
A workflow that can expand into similar photos or large videos only when you choose that broader pass.
When a focused duplicate workflow is enough
A focused duplicate workflow is enough when the library problem is mostly exact repeats from imports, saves, edits, or accidental re-downloads. In that case, you do not need a broader cleanup decision first. You need a clean duplicate pass.
When the job is broader than duplicates
If the library is really full of similar shots, bursts, screenshots, and large videos, then a duplicate-only answer is too narrow. That is when the comparison question becomes broader than one duplicate feature.
If you want the broader shortlist, open best iPhone cleaner apps. If you want the feature-level route next, continue to duplicate photos.
The best duplicate-photo cleaner does one thing very well: it makes exact waste obvious without turning the pass into a harder photo judgment task.