The Océ Gemini Instant Duplex technology forms the heart of the Océ VarioPrint 6160/6200/6250 printer line. Since duplex productivity is key, the main technological issues that have been solved during development were architectural and robustness related. Robustness, in our philosophy, not only means large lifetimes of parts and consumables, but also a robust, constant and consistent performance and print quality of both print processes for various media and environments. To realize the necessary print quality robustness, we have made various innovations in the OPC, LED-printhead, transfer efficiency, transfuse robustness and process control.In this contribution we will first describe the essentials of the printing process of the Océ VarioPrint 6160/6200/6250 printer line. After that, we elaborate a little further on the influence of ambient humidity conditions on the OPC-belt and its production process. Finally, the transfer-transfuse process and the heat interaction with the OPC-belt is discussed. These items represent two related print quality problems that had to be solved in order to realize a productive duplex print engine on an integral level.