An image forming apparatus and a method of controlling the same capable of preventing an unnecessary repetitive performance of a scan task by sensing in advance a succeeding scan command generated with respect to a document which has already been scanned and using a previous scan result without an additional scan task are provided. The method includes generating and storing a first image through a first scan task according to a first scan command, determining whether a document at the first scan task is replaced by a new document using document sensors if a second scan command is generated, and using the first image as a result of the second scan command without performing a second scan task according to the second scan command upon determining that the document at the first scan task is not replaced by the new document.