1. A method of operating an image-based check processing system to detect a double feed condition of carrier envelopes, the method comprising: attempting to read a magnetic ink character recognition (MICR) codeline from a first predefined area of an image of a document item; based upon the attempted read of a MICR codeline from the first predefined area of the item image, determining if the document item is a carrier envelope; concluding that the document item is not a double-fed item when a determination is made that the document item is a carrier envelope; and concluding that the document item is potentially a double-fed item when a determination is made that the document item is not a carrier envelope.