In single-pass duplex scanning, both sides of the piece of paper are scanned at the same time. Single or simplex scanning means only one side is scanned at a time. Obviously, duplex scanning is a time saver if you have multiple double-sided pages through scanning speeds can be slower. In single-pass duplex scanning, both sides of the piece of paper are scanned at the same time. Single or simplex scanning means only one side is scanned at a time. Obviously, duplex scanning is a time saver if you have multiple double-sided pages through scanning speeds can be slower.