Years ago I worked for a document imaging company dealing with legal documents.
Scanned document images can be run through an algorithm to produce a unique number—a hash code. If there is even only one pixel difference between two documents, they will produce different hash codes.
The documents don’t need a unique pre-printed number. Their uniqueness can be measured by creating hash codes of the scans.
BTW, this could still be done to determine how many duplicate votes were scanned—as long as you have a paper ballot to scan.
I didn’t say a unique number. I said a unique identifier. A hash code would be a unique identifier. Please pay attention to what you read next time.