A system for detecting double-feed flat item conveyed in a mail processing machine, including a detection for directing a beam of radiant energy toward the moving flat items, scanning them with the beam and receiving at least a portion of the beam of radiation reflected from them. The detector includes a triangulation sensor for providing an output proportional to the position at which the reflected portion of the beam is received, and means for determining from the output, the distance (d) between the radiation source and the point of reflection of the beam on the moving flat items, and providing a signal (S) representative of said distance; and a controller configured to receive the signal (S) and generate an output signal (V) indicative of a flat item profile and a double-feed condition when it detects a significant break point or slope change of the signal from a first direction to a second direction.