You can look into the Photos Library package, if you quit Photos, then ctrl-click the Photos Library and use the command "Show Package Contents". This will allow you to look inside and inspect the stored files. The original image files will be in subfolder "originals". Be very careful not to modify anything inside. You might damage the library. And there is nothing useful you can do anyway with the internal parts of the library.
In macOS 10.15 Catalina the internal organisation of the Photos Library package has changed. Photos is no longer storing the original image files with their original filenames but renaming them with unique new names. When you import duplicates, the new, unique filenames may differ. It is impossible to find files by their original filename.
See: How Photos 5.0 on Catalina Manages origin… - Apple Community
To see, if two images in photos are duplicates, you can compare the original filenames in the Info. The second line of the Info for a selected photo is showing the filename.
If a photo does not have a title, you can see the original filename below the thumbnails, when you are viewing the photos in "All Photos" or in a standard album. Just enable the option "View > Metadata > Titles".
How to view the titles or filenames below the thumbnails in Photos 5.0 - Apple Community
