1. An image forming apparatus comprising: a combining unit configured to combine a document image and a coded image; a setting determining unit configured to determine whether a setting for combining a document image including a previously combined coded image and an other coded image is made when a reproduction of the document image is instructed; and a prohibiting unit configured to prohibit the combining unit from combining the document image including the previously combined coded image and the other coded image according to the setting when the result of the determination indicates that the setting is made.