Most film has datecodes. So if the dupe negatives were done much later, with new stock it would be possible to fiure out the actual date of the dupe negative.
http://www.film-center.com/dates.html
That's mostly about movie film, but still film usually has codes too.
Much duplication isn't done all that well, and the image loses a bit of focus, and often some contrast as well. There are modern systems that do much better, laser scans and such, but the old ways generated a slightly lossy image even with a good operator. tere's a tiny bit of distortion caused by the film substrate itself. A contact print emulsion to emulsion would be the way to go, but for most places it would be too much work. The smaller the original, the easier it would be to tell. Blowing up from a 35mm negative to an 8x10 negative would take a some skill to have it come out clearly. Doing it from a professionals medium or large format negative would be easier, and if the original was a very large format camera like one that actually took 8x10 negatives it would be hard to tell the dupe since the person working to make the duplicate would be quite skilled.
Steve B