I can't believe we're still discussing duty cycle and square waves? - this is an analogue RF multiplier stage!. It accepts a sinewave input to a class C amplifier, which 'roughly' works as a switch - this doesn't mean it outputs a beautiful square wave, of any duty cycle - there's no need for it.
Essentially you're not looking for harmonics, although they may assist a little as well, you're 'ringing' the tuned circuit in the collector - so it will multiply quite happily by two or by three, and even higher if required (although two or three are by far the most common).
For anyone at all interested?, and not confused by irrelevant duty cycles?, 144MHz (2M) amateur radio transmitters commonly used 12MHz crystals so required multiplying twelve times. This was done with two doublers and a tripler - all identical looking class C stages with tuned collectors (to different frequencies obviously!).
Another (less common) crystal was 8MHz, so times eighteen (x3, x3, x2).