Photos folder list does not have a "Duplicates" folder on my older iMac running Version 16.6 (17615.3.12.11.3, 17615) I have about 5,000 photos in my library and numerous albums with known duplicates. I have yet to see the "Duplicates" folder name.
Earlier Mac models
In addition to den.thed's reply albums are like playlists in iTunes. You can have a song in multiple playlists in iTunes but only once in the library. The same goes for albums.
Images in Albums are not duplicates, they are just alias shortcuts back to the main Library.
Version 16.6 (17615.3.12.11.3, 17615) is a version of Safari browser. Are you looking for a duplicated folder in Safari?
Photos folder list does not have a "Duplicates" folder on my iMac.
