Image forming apparatus and method of controlling the image forming apparatus

An image forming apparatus comprising:

• a first paper feeding unit;
• a second paper feeding unit;
• a fixing unit configured to fix a developer image transferred onto a sheet;
• a conveying path configured to convey sheets from the first paper feeding unit and the second paper feeding unit to the fixing unit;
• a receiving unit configured to receive an execution instruction for duplex printing for continuously executing a first operation for printing an image on one surface of a sheet and a second operation for printing an image on the other surface;
• an acquiring unit configured to acquire, in the first operation in the duplex printing, information concerning presence or absence of an image on the other surface of the sheet conveyed through the conveying path;
• and
• a processor configured to change, in the first operation in the duplex printing, if a result of the acquisition by the acquiring unit concerning the sheet fed from the first paper feeding unit indicates that an image is present, a paper feeding unit from the first paper feeding unit to the second paper feeding unit and performs control for re-executing the duplex printing.