Every signal will appear with a delay on the input 1 comparing to input 2 (see the schematic above) of the XOR element.
The delay is equal to the propagation time of the NOT gates.
Due to the delay the XOR element will generate a pulse on every change of the input signal.
I believe ( please correct me if I am wrong ) there will be
XNOR gate if two NOT gates are being used i.e. effectively
XNORing the input and its replica. If XOR gate is used the
input signal and its inverted will be XORed.
Using two NOT gates to one input of XOR is doing nothing.
It is just helpful in generating a delay but not doubling
the output frequency.
Instead attaching a buffer to the input of XOR can buy some
time and produce a significant delay which can double the