We present a broadband and polarization-insensitive terahertz metamaterial absorber formed from double-layered same-sized patterned square patches on top of a metallic ground plane. The combination of the two same-sized metallic patches gives rise to an absorption bandwidth of 380 GHz with absorption over 90%. The full width at half maximum of this device with respect to the central resonance frequency can be up to 33%, which is double that of a single-layered structure. The mechanism of the broadband absorber is attributed to longitudinal coupling between layers, and a hybridized charge distribution model is proposed for analyzing the origin of the resonance bandwidth. The proposed absorber has potential applications in detection, imaging and stealth technology.