Automatic document feeder and method for duplex scanning a document utilizing the same

An automatic document feeder includes a first path extending from a feeding tray to a scanning module, a second path extending from the scanning module to a reversing roller unit, and a third path extending from the reversing roller unit to an intermediate portion of the first path. A document is moved from the feeding tray onto the reversing roller unit via the first and second paths for a first scanning. Just before the document is moved through the second path, the reversing roller unit is driven to allow the document to move therethrough. Subsequently, the document is reversed on the reversing roller unit to move through a looped passage defined by the third path, a portion of the first path downstream of the intermediate portion, and the second path for a second scanning. The reversing roller unit is operable to allow two opposite edges of the document to move thereon in different directions.