Three earthquake sequences, each with two main earthquakes of almost the same magnitudes (M_L from 5.9 to 7.0 with differences less than 0.1), have recently been observed in Taiwan. The two largest earthquakes in each sequence occurred with a short delay time between them, were strikingly similar in terms of magnitude, location and/or focal mechanism and are referred to as doublets. They were markedly distinct from typical single mainshock sequences. Our estimated static stress field generated by the first shock in the doublet shows that the second shock and most of their aftershocks were located within a region where static stress increased substantially. Thus, a possible explanation for earthquake doublet is that seismic energy for each shock had accumulated independently within adjacent crustal volumes, separated by an asperity, and that the second shock is triggered by the increased static stress after the first one. An important implication of doublet sequence is that works by emergency response teams after the first earthquake could be made more hazardous by the second.