The polarity of voltage applied between the dees are changed itself when the electron completes its revolution in one dee. This means that the time required to completes one half cycle in the dee is equal to the time of change in polarity of the voltage, this is the resonance condition. And as the frequency of the radio frequency field is doubled then violation of resonance condition will be there as the time required in changing the radio frequency filed will be halved. And, due to this there will be alternate acceleration and deceleration of the electron which will make it follow the same path i.e. radius of its path in the dees will be same.