Two different directions of work
Two-sided printing turns a digital file into a sheet printed on its front and back. Two-sided scanning captures both sides of an existing paper original. A machine may perform the first task automatically without performing the second automatically. A specification for duplex printing therefore does not answer the question about your scanning pile.
An automatic document feeder also does not, by itself, establish automatic two-sided scanning. It feeds a series of originals through the machine. Look for explicit support for scanning both sides in the specification for that exact model. Suitable feeders can capture the sides in different ways; for your initial decision, establish whether you need to turn the sheets yourself and which originals are supported.
Compare using a small numbered sample
Prepare a few sheets with clearly numbered sides and mark the top edge. Ask for them to be scanned into a PDF, then inspect the order, orientation and completeness of the result. A sample resembling your real paperwork tells you more about the workflow than a demonstration using an unrelated document.
Include a mixture of one-sided and two-sided pages rather than only a fully printed stack. If the software removes blank pages, inspect the result particularly carefully. An apparently blank back might contain a faint annotation. Automatic removal is useful only when it preserves the information that matters to you.
For occasional two-sided scanning, manually turning a stack may be acceptable. In that case, judge how clearly the software guides you through the second pass and whether it combines the pages in the intended order. Include those handling steps when comparing the workflow instead of measuring only the speed of one pass through the feeder.
Check copying as its own task
If you also want paper copies of two-sided originals, test that precise copying sequence. Establishing that a machine can separately print and scan is not enough to demonstrate the complete task. The relationship between a one-sided original, a two-sided original and the desired output should be understandable in the controls. Record a useful setting if several people will repeat the job.
Give the purchase comparison three separate lines: automatic duplex printing, automatic duplex scanning and the copying sequence you need. A missing function then remains visible. This is more informative than a single tick beside the word duplex and avoids discovering that the step you perform most frequently still requires manual handling.
