A printing apparatus comprising an image forming system, a sheet-feeding system, a sheet-transporting system, a sheet-controlling device, a sheet-detection device, a first driving system, a second driving system and a controller is provided. The sheet-controlling device and the sheet-detection device are disposed between the sheet-feeding system and the sheet-transporting system. The sheet-controlling device controls an entry of a sheet into the sheet-transporting system. The first driving system drives the sheet-controlling device, and the second driving system drives a transporting mechanism of the sheet-transporting system. The controller upon receiving an error signal from the sheet-detection device signals the sheet-controlling device to inhibit the entry of the sheet into the sheet-transporting system and signals the transporting mechanism to continue its operation. A duplex printing method is also provided.