I scan my student's assignments. There is a big printer with scanning function in our department where you can just put in the papers and it scans all of them at once.
Moreover, I do warn them beforehand that I will scan the papers and check if one makes a complaint. This has the advantage that the students will not cheat (in this way) and saves me the time from finding the student's scan in a huge pdf file and comparing the two versions.
Of course, to make scanning practical, you should tell the students not to staple their assignments (and only staple them afterwards), so as to be able to feed an entire pile of papers into the xerox.
