4. The image processing method according to claim 3 , wherein, when the subsequently-read document is classified as non-similar, the stored feature vectors and at least the first identifier are erased from storage, the subsequently-read document classified as non-similar is treated as the first document, and all steps of the image processing method are repeated.
3. The image processing method according to claim 2 , wherein, when the number of stored identifiers reaches the predetermined extent, the stored first identifier is assigned to the subsequently-read document or the subsequently-read document is classified as non-similar based on the calculated image similarity.