Public Photocopiers - Double-sided copying.
It is possible to do this but it is awkward and more likely to produce a jam.

Putting a piece of paper through a copier twice is more likely to cause a jam when the paper has just come out. I think this is because of the tremendous amount of heat used to bond the ink to the paper and the resulting curl on the paper. So if the page is not nice and flat like a fresh piece...it doesn't go through the copier as well.

To do double-sided, there is a feeder tray on the right side of the machine. It is folded up, so must first be pulled out and down to open it.
The risk here is, the user has no way on knowing 'how' they must place the sheet in the feeder so the copies will come out on the correct side or position on the paper. So tell them they can do it if they don't mind wasting a few copies on 'experimenting' with how to position the paper.