A printing apparatus and method for duplex printing a document having a show-through watermark wherein the watermark is comprised of first and second images on both sides of the document. The images comprise stoclustic halftone screens having a substantially common registration wherein the second image is relatively shifted from the first image for forming an associated overlay pattern between the first and second images resulting in a recognizable pattern comprising the watermark upon illumination by a show-through light source. The watermark is unrecognizable from either image alone upon illumination from a front light source. The relative shifting can be implemented by either phase, frequency or angle shifting between the front and back side images.