An experimental investigation was performed with two aims: (1) to clarify the existence of the dual leading-edge vortices (i.e. two vortices with the same sense of rotation located close to the leading edge above the leeward wing surface) observed on flapping wings in previous studies; (2) to study systematically the influences of kinematic and geometric parameters on such a vortical structure. Based on a scaled-up electromechanical model flapping in a water tank, the leading-edge vortex (LEV) cores were visualized via dye flow visualization, and the detailed sub-structures of LEV were revealed through digital particle image velocimetry (DPIV) with high spatial resolution. Five wing aspect ratios (AR) (1.3, 3.5, 5.8, 7.5 and 10), eight mid-stroke angles of attack (alpha(m)) (10-80 degrees), and six Reynolds numbers (Re) (160-3200) were examined. In addition, the well-studied case of the fruit fly Drosophila was re-examined. The results confirm for the first time the existence of dual LEVs on flapping wings. The sectional flow structure resembles the dual-vortex observed on non-slender delta wings. Insensitive to AR, a dual LEV system such as this could be created when alpha(m) and Re reached certain high levels. The primary vortex was attached to the wing, while at the outer wing the minor vortex shed, generating a same-sense vortex behind.