Duplex printing is not supported on the Color LaserWriter 12/600 PS. Using duplex printing can cause complications with the printer.
The toner material, if already applied and fused to the paper, can become sticky if passed through the fuser assembly again. It could then possibly adhere iteself to the heater rollers in the fuser making it very difficult to remove. Damage to the fuser's heater rollers is possible.
Once the paper passes through the fuser assembly, and moisture is evaporated from it by the fuser's heater bulbs, the paper may tend to curl slightly. This may also cause potential paper jams or misfeeds.