Spending too much time in photo archives, I would often come across prints that looked like duplicates, like this famous picture of Mark Twain or this one of Roosevelt in Yellowstone. I did not think much of them, assuming that was just an industry standard that photographers used back then for copies – I learned recently however that these were special prints called stereographs, and when viewed with proper glasses would produce a crude 3D effect (it is essentially the same concept behind the View-Master, a toy that many of us probably had when we were kids).
The Library of Congress maintains large collections of stereographs, including many from the early 20th century when it was a big industry for photographers. Some favorites:
