ORIGINAL: BarracudaHockey
RC recievers mix a signal with the incoming signal. It produces 4 outputs, the 2 original signals, the sum of the two and the difference of the two. Lower freqency signals are easier to work with the reciever extracts the difference of the two and uses that to decode the servo position information.
Dual conversion does this twice eliminating some types of interference.
Superheterodyne recievers are subject to image frequency interference. The image frequency is another frequency that results in the sum frequency being listened to by the intermediate frequency section. By converting twice, it is easier to reject the image frequency.