The Apple .photosfile is an App Bundle, meaning it is really a folder.
If you look into the folder, you will see that it consists of other folders, and files.
Duplicati treats this as any other directory and can make a backup of this by backing up the details. Inside are various .sqlite files which can also be backed up. You may want to exclude files ending in -journal as these are temporary transaction files created by SQLite, and are not likely to be useful.
When you restore, the bundle will be restored in full and work the same. The only thing that is not currently handled perfectly is Apple extended attributes and I do not know if Apple Photos is relying on these.