The duplex scan that comes out backwards

A single-sided sheet feeder cannot scan both sides at once. So the standard trick is to scan the stack, flip the whole stack, and scan it again — which gives you two files and a small puzzle.

Why the second file is reversed

Think about what physically happens. You feed a fifty-sheet stack face down: the scanner takes sheet 1, then 2, then 3. You get fronts in order.

Now you pick up the whole stack, turn it over, and feed it again. The sheet that was on the bottom is now on top. The scanner takes sheet 50's reverse first, then 49, then 48. You get backs in reverse order.

Neither file is wrong. They are both faithful records of the order the paper went through the machine. They just need to be woven back together with one of them running the other way.

The reassembly

You want front 1, back 1, front 2, back 2, and so on. Since the backs file runs 50, 49, 48…, reversing it first gives 1, 2, 3… and the two can then simply alternate.

That is a single operation — interleave, with the second file reversed — and it is worth knowing it exists, because the manual alternative is fifty pages of drag and drop and one transposition error that nobody notices until a page is cited in the wrong place.

The two things that go wrong

Odd page counts. If the last sheet was blank on the back, some scanners skip it and some do not. If your files are 50 and 49 pages, alternating naively will run one short and quietly drop a page, or stop early. Interleaving should keep taking turns until *both* files are exhausted, not stop at the shorter one.

Blank reverse sides. Duplex scanning a stack where half the sheets are single-sided produces a file full of blanks. Detecting and removing them afterwards is easy — but do the detection *after* interleaving, not before, or you will destroy the alternation that makes the reassembly work.

Getting the order right the first time

If you are about to scan a large stack and have a choice, feeding face-up rather than face-down changes which file ends up reversed, and some drivers have a "reverse order" option that handles it in the scanner. Either is less error-prone than fixing it afterwards.

But if you are holding two files already, you do not need to re-scan anything. The information is all there. It just needs putting back in the order the paper was in.