Based on the Berakdar theory,this work investigates theoretically the triply differential cross sections for electron impact ionization of Li+(1s2) at an energy of 100eV in coplanar doubly symmetric geometry,i.e.,equal energies and equal emission angles for the two electrons in the final state.Dominant ionization mechanisms are identified and the contribution of different scattering amplitudes and the major role of initial channel Coulomb field are discussed.It turns out that the cross section in doubly symmetric geometry is strongly influenced by the interference of these amplitudes and the initial channel Coulomb field.