Change AC signal frequency without changing the voltage
If you can find a way to control the the voltage output and power, a tried and tested way of frequency doubling is to multiply the signal by itself using an analogue multiplier. It does depend on a few other details you have not provided: Is it a pure sine wave, or does the source contain signals at more than one frequency?

Since 

sin(t) * sin(t) = 1/2 - 1/2 cos(2t)

it is clear that multiplying a pure sine wave (i.e. of a single frequency) by itself, results in a wave with a frequency of twice the original. i.e. the factor 2 in cos(2t). Since the frequency is low, I'm sure a standard analogue multiplier will work well.

You may find the following to be useful:
http://en.wikipedia.org/wiki/Analog_multiplier

Some actual chips and application notes are mentioned here:
http://www.analog.com/en/search.html?q=analog%20multiplier