The frequency doubler uses only one !C. Like other doublers, this circuit uses both the rising and falling edges of the input signals to produce digital pulses, thus effectively doubling the input`s frequency. Without the rc networks at IC1 inputs, IC1 would not produce any output pulses. However, the rc networks delay one edge with respect to the other. The A input lags the B input for positive-going edges, and the B input lags the A input for negative-going ones.
You can vary the output duty cycle from 0 to 100% by varying R3. IC1"s minimum output pulse width defines the maximum frequency of this circuit.