when forming an image on both sides of set number of sheets of paper , excess paper which remains in an intermediate tray caused by an overlapped feeding of the paper is guided into a bin which has not been in use for the image forming of the above set number of sheet . further , when some sheets of the paper discharged from the intermediate tray are transferred in an overlapped condition and the paper in the intermediate tray becomes insufficient , the image forming operation corresponding to the insufficient number of sheet of the paper is discontinued , and the sorting operation for the next original starts from a sort bin skipping over some sort bins which should have received the uncopied paper because of overlapped feeding .