According to one example, there is provided a method of duplex printing. The method comprises printing recto pages and associated recto checkmarks on a first side of a web and printing verso pages and associated verso checkmarks on a second side of a web, the length of each successively printed recto and verso checkmark varying in accordance with a respective recto and verso checkmark length sequence, determining the length of successive printed verso and recto checkmarks and determining their sequence position in the appropriate checkmark length sequence, and determining whether a print error has occurred based on the determined recto and verso checkmark sequence positions.