How do I organize and delete duplicate photos on iPhone's Photos app?

The Photos app itself can find and sort duplicates.

Go to the Collections tab, and scroll down to the Utilites section, then tape on Duplicates to see the list if duplicates the photos app has identified. You can then delete them or merge them.

click here ➜ Merge duplicate photos and videos on iPhone - Apple Support

First, and very important: stay away from duplicate finder apps! Many of these apps don't understand how Photos works, and they have been known to really mess up the Photos database.

I do all my editing and organizing with Mac Photos, and I know that here are a couple of reliable duplicate finder apps for the Mac. One app that works well on the Mac, PhotoSweeper, has an iOS version, PhotoSweeper Mobile for $2/month, but I haven't used it on my iPhone.

As Phil0124 said, the best thing is to use Photos' own duplicate finder. It "merges" duplicates to get what it thinks is the best of both.

How are you importing photos on your device? Are you asking about Photos for Mac or Photos on a mobile device (iPhone, iPad, etc.)?

And what kind of duplicates are you having on your device? Are there exact duplicates (meaning identical image files imported twice or more) or different versions or similar photos?

The Photos.app is usually preventing us from importing the same photo twice and warning us, if we are about to import an image or video again, that is already in our library. There should not be any exact duplicates in Photos at all, unless we duplicate a photo intentionally. But Photos will not warn us, if someone keeps sharing the same photo over and over again with us, with different filenames or different sizes or different creation dates. But the built-in duplicate detection in Photos will collect different versions of the same photo and you can let Photos merge them automatically or select the keepers yourself. I am usually checking the metadata of of the photos in the Duplicates albums, because they may differ, if I already have edited the photos. And the Photos.app cannot know, which date or title is then I want to keep, when it is merging the duplicates.

if you are having duplicates on a Mac caused by importing photos from an iPhone using a USB connection, you should check the Settings on the iPhone. There is a problem, if your iPhone is using the space saving High Efficiency format (HEIC od HEVC) but you are importing these items converted to a more compatible format. This can disable the duplicate detection. on import and you might reimport the same photos over and over again. Avoid the format conversion when importing (Using HEIF or HEVC media on Apple devices - Apple Support (EG)), as it is disabling the duplicate section on import. Also do not import via USB if the iPhone is syncing with iCloud Photos. You might get random results, because the downloaded local copies on the iPhone or iPad may have different file creation dates from versions you have previously downloaded.

Where are you seeing the duplicates? In the Photo app's interface?