1. An image forming apparatus having a first sheet holding unit and a second sheet holding unit, the image forming apparatus comprising: a reading unit configured to read a first image on a sheet fed from the second sheet holding unit, and generate image data corresponding to the first image; storing unit configured to store the image data generated by the reading unit in a memory; a detecting unit configured to detect a sheet type of sheet fed from the second sheet holding unit; a printing unit configured to print a second image onto a sheet fed from the first sheet holding unit or a sheet fed from the second sheet holding unit; and a control unit configured to, when the printing unit is to print the second image onto a sheet fed from the second sheet holding unit, allow the printing unit to print the second image onto a sheet fed from the second sheet holding unit in a case where the sheet type detected by the detecting unit is a first sheet type, and restrict the printing unit from printing the second image onto a sheet fed from the second sheet holding unit in a case where the sheet type detected by the detecting unit is a second sheet type, wherein, when the printing unit is to print the second image onto a sheet fed from the first sheet holding unit, a sheet type of a sheet fed from the first sheet holding unit is not detected by the detecting unit before the printing unit prints the second image thereon.