1. An image data output processing apparatus which carries out an output process with respect to input image data, the image data output processing apparatus comprising: an image matching section that (i) extracts features of an image of the input image data and (ii) carries out a determination as to a similarity between the image of the input image data and an image of a reference document based on a comparison made between the features extracted and features of the image of the reference document stored in advance; and an output process control section that controls the output process with respect to the input image data based on a determination result made by the image matching section, wherein the image matching section carries out a matching of the image of the input image data by use of a storage section which stores (i) features of each of images on a front side and a back side of a duplex reference document and (ii) an index for identifying a duplex document including the reference image, the index being stored so as to correspond to an index for identifying each of the images on the front side and the back side of the duplex document, the image matching section further including: a voting process/candidate list creation section that extracts, in a case where the input image data is of a duplex document, for each of the images on the front side and the back side of the duplex document, (i) a reference image index of a reference image having a similarity which is not less than a predetermined value from the storage section and (ii) a reference document index corresponding to the reference image index from the storage section, so as to create a front side candidate list and a back side candidate list; and a duplex document determination process section that determines, in a case where each of the front and back side candidate lists created include a reference image index that corresponds to an identical reference document index, that the images on the duplex document of the input image data are similar to the reference images that correspond to the identical reference document index, respectively.