How to create a frequency doubler circuit using only flipflops/ Digital elements?
Doublers are possible using digital circuits, the following diagram is one such example. The trouble with it is that it relies on propagation delays in delay chains in order to generate the doubled frequency. This means that you don't get a guaranteed duty cycle, it will vary depending on how long your chain is. The clock will probably also be fairly jittery.



In fact, such a circuit would probably work as a frequency doubler even without the register - with just the input clock connected directly to the delay chain.