A system for printing to recycled paper sheets, by scanning a sheet for existing marks, comparing them with the intended output, and printing upon determining the suitability of printing the intended output to the sheet. When recycled paper is used for printing, the intended output is sometimes printed on the wrong side of the sheet, resulting in wasted paper, toner, and other resources. This system avoids printing to the wrong side of sheets, and enables adjustment of intended output for better result. Sheets can be reversed, rotated, and discarded, and the intended output can be adjusted by shrinking, moving, and rotating. Methods also include previewing the output and the existing marks together, low-resolution processing, printing shading patterns on existing marks, rearranging scaled-down pages of N-up printing, Duplex support, collision detection with user-customizable, threshold overlap value and separation distance, and estimating separation distance by boundary-augmentation of alternating 4- and 8-neighbors.