In this study,we investigate the generation of twin γ ray beams in the collision of an ultrahigh intensity laser pulse with a laser wakefield accelerated electron beam using a particle-in-cell simulation.We consider the composed target of a homogeneous underdense preplasma in front of an ultrathin solid foil.The electrons in the preplasma are trapped and accelerated by the wakefield.When the laser pulse is reflected by the thin solid foil,the wakefield accelerated electrons continue to move forward and pass through the foil almost without influence from the reflected laser pulse or foil.Consequently,two groups of γ ray flashes,with tunable time delay and energy,are generated by the wakefield accelerated electron beam interacting with the reflected laser pulse from the foil as well as another counter-propagating petawatt laser pulse behind the foil.Additionally,we study the dependence of the γ photon emission on the preplasma densities,driving laser polarization,and solid foil.