On an iPhone, when you add photos to an album, those photos are not duplicated within the album itself. The photos in your albums are essentially pointers or references to the original photos stored in your photo library.
For example, let's say you have a photo library on your iPhone with 100 photos. If you create an album called "Family Vacation" and add 20 photos to that album, those 20 photos are not physically copied or duplicated. Instead, the album contains references to the original photos in your library.
This means that the photos you see in your album are the same as the ones in the "Recent" album under the Albums tab or in the "All Photos" section of your photo library. Any changes you make to a photo, such as edits or deletions, will be reflected across all instances of that photo, whether it's in an album or the main library. So, you don't have to worry about duplicated photos taking up extra storage space on your iPhone.