DB Labs
The Short Answer
Every duplicate photo takes the full size of the original. A 5 MB HEIC photo duplicated once takes 10 MB total. Duplicated five times across burst, AirDrop, iCloud sync, and messaging apps, it can take 25 MB or more. On the average iPhone, duplicates account for 10 to 20 percent of total photo storage, often several gigabytes. The Photos app has a built-in Duplicates album under Utilities that detects exact matches and offers a one-tap Merge. For near-duplicates like burst sequences, Swype Photo Cleaner handles triage in minutes.
Why Duplicates Appear
Duplicates happen in several ways on iPhone. AirDropping a photo from a friend creates a new copy. Saving an image from Messages saves it as a fresh file. Burst mode captures 10 similar shots per second. iCloud sync hiccups sometimes duplicate entire albums. Editing a photo and saving a copy doubles it. Over a year, the average user accumulates hundreds of duplicates without realizing it.
How Much Space They Take
Duplicates take the full size of the original, not some compressed thumbnail. A library with 500 duplicates averaging 4 MB each uses 2 GB purely on duplication. For 4K videos the number is far worse: a single duplicate 4K clip can eat 500 MB to 1 GB.
Finding Duplicates
Open Photos, tap Albums, scroll to Utilities, tap Duplicates. Apple's duplicate detection finds exact matches and very close variants. Tap Merge on any pair and Photos keeps the best version, folding the others. The operation is reversible via Recently Deleted for 30 days.
For burst-similar or near-duplicate shots that Apple does not flag, Swype Photo Cleaner shows you one photo at a time so you can choose which to keep.
Preventing Future Duplicates
A few simple habits prevent most duplicates: turn off Save as Copy when editing in third-party apps, do not save photos from iMessage that you already have in Photos, use the built-in AirDrop receive behavior (which does detect duplicates in some cases), and avoid burst mode unless you need it. A weekly quick Duplicates album check catches anything that slips through.