Image forming apparatus and image forming method

An image forming apparatus comprising:

• a sheet feeding mechanism configured to feed one or more print sheets;
• an image forming mechanism configured to:
• form a first image on one surface of the one or more print sheets,
• allow the one or more print sheets to pass through the image forming mechanism, and
• form a second image on the other surface of the one or more print sheets when the one or more print sheets that have passed through the image forming mechanism are flipped back;
• a double-sided transport mechanism configured to:
• flip back the one or more print sheets on which the first image is formed, and
• guide the one or more print sheets to the image forming mechanism again;
• a post-processing unit mounter on which a post-processing unit, that performs post-processing on a leading end or a trailing end of the one or more print sheets that are to be discharged, is mountable;
• and
• one or more controllers configured to control a print job, wherein
• when executing the print job, in which at least a sheet of the one or more print sheets is an index sheet and a post-processing function is set, the one or more controllers determine (i) whether the post-processing is to be performed, (ii) an order of the first image and the second image to be formed, and (iii) whether to flip back the one or more print sheets by using the double-sided transport mechanism according to a type of the post-processing performed by the mounted post-processing unit is to be on the leading end or the trailing end of the one or more print sheets.