Searching for an app to delete duplicate photos from iPhone

The default Apple Photos app includes a duplicate finder. Is that not enough?

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

I have experience with other apps, but someone else may be able to suggest a specific app.

I think it's worthwhile to advise you to be very very careful about apps claiming to "fix up" your Photos isn some way. The Photos database has many parts, and if any one is changed in some way, then other parts have to also be changed to include the addition, or major corruption can occur. Apps that do this safely are much less common than apps that don't care. I would want a good backup for my pictures before trying to use any 3rd Party app.

We know that PhotoSweeper works properly on the Mac Photos Library, and there is a Mobile version on the iPhone App store -- its $10/year, which, amazingly, is not bad at all.

But I'd have a backup of my Pictures!

Duplicates is in the Utilities section in the Sidebar.