Abstract: A duplex printing apparatus has a printing unit for consectively printing images on the first sides of paper sheets and on the second sides of paper sheets. A reversing unit is mounted adjacent to the printing unit for receiving the paper sheets discharged from the printing unit and for consecutively reversing the paper sheets. A duplexing unit is mounted adjacent to the reversing unit and the printing unit, for receiving the reversed paper sheets from the reversing unit and for feeding the reversed sheets back into the printing unit for printing on the second sides of the sheets. A control unit is coupled to the printing unit, the reversing unit, and the duplexing unit. The control unit determines an interleave number that equals the maximum number of sheets with images printed on their first sides that can fit within a duplex feed path, so that each consecutive sheet is spaced a sufficient distance apart to interleave a new sheet therebetween.