Normally, when a signal is described as PWM, then the duty cycle is important, and the frequency is reasonably constant. However in your case it seems that the duty cycle doesn't matter, but the frequency changes a lot.
A simple frequency doubler would produce a pulse on the rising and faling edge of the signal. Something like **broken link removed** but you should use Schmitt trigger inputs. That would give the same pulse width at all frequencies so the duty cycle would be much less than 1% at low frequencies.
If you want to a constant mark-space ratio, you would need a significantly more complicated solution.
Can you say anything more about the application?